With Apple having announced some new machines with really nice price points within the last week, I've been asked by some friends what my suggestions would be for copying the data to a new machine and removing data from the old one. Here, I'll try to put forth my current thoughts on these two issues.

Some of these instructions may vary depending on whether you have a machine with Firewire 400, Firewire 800, or no Firewire. Further, if one machine has Firewire 400 and the other Firewire 800, the instructions differ by one more cable.

Generally speaking, I'm a big fan of Apple's Migration Assistant. This isn't to say that there's no argument for starting from scratch each time, but the frustration of finding out you forgot to bring something over from your old machine is pretty big.

Prerequisites

So, what should you do in preparation? Here are the steps that I'd suggest prior to hooking the machines together for this action. Frankly, do these even before you turn on your new machine.

1. Make sure you have an up-to-date backup. Time Machine is great for this, but if you don't have time Machine running, programs like Carbon Copy Cloner or SuperDuper! make copies of your disk to a new hard drive quickly and easily and can serve as a good alternative.
2. Deauthorize iTunes. Launch iTunes on the old machine, select Deauthorize Computer... from the Store menu. This will make sure that the machine you are getting rid of doesn't have a license to your video/audio content. Mostly this is important because it is inconvenient to reset the limited number of machines you can do this with when you run out of licenses. However, if you have to do so, there are procedures for Deauthorizing All Computers available from Apple.
3. Deauthorize all Adobe paid-for products. This is an absolute Must if you own Adobe commercial products. Unlike Apple's iTunes, there is no real way to get things cleared up if you don't deauthorize your copy of Photoshop or Illustrator. If you don't catch this before nuking the existing computer's hard drive, it can be a real pain in the neck to do it later, and it will require a complete reinstallation of the software on the machine before it happens.
4. Make sure you have your serial numbers. Most, but not all, products carry their serial numbers around (if they have them) in their preferences file. However, some programs, notably Apple's Pro products (Aperture and Final Cut Products) don't retain the entire serial number, so you will need the original number from your box. There's pretty much no way around this.
5. Dig up your original installation media that came with your old computer and (if you have upgraded it), the media for the newer version of OS X that you have installed on it currently. Now: please keep in mind that you can give away your license to Leopard (as an example) with the machine that it is installed on, but only if that is in compliance with your license agreement. In particular, you can't give a family-pack copy of OS X to somebody who doesn't share the same physical address, unless they are getting the entire copy (i.e. all your other "family" machines are licensed either by having the OS included on their install disks, or because you have separate upgrades for them. Of course, you may never give away a copy of OS X that you have only one license for if you are keeping it running elsewhere.
6. Find an appropriate cable (or not). For any computer that has a Firewire interface, you need to find a cable that can go between the old and new computer. So, if you have a MacBook and you are upgrading to a MacBook Pro, you will need a Firewire 400 to Firewire 800 standard. If you have a MacBook Air, you'll have to migrate over the network.

Now you're ready to start the migration process

Migration (Thunderbolt/Firewire)

Here are the steps to follow to migrate your machine to the newer machine.

1. Start the new machine. Go through the basic process and eventually, you will reach the page where it asks you if you want to Migrate existing data. This is called the Migration Assistant and if you somehow miss it, you can run it after you perform the basic install by double-clicking on Migration Assistant in the Utilities folder under Applications
2. Answer the "How do you want to transfer your information" with From another Mac
3. Plug the Thunderbolt or Firewire cable in to both machines
4. Answer the Migrate from Another Mac question with Use Thunderbolt (or Use Firewire as appropriate)
5. Follow the instructions by restarting your other mac while holding down the T key. Once the old machine is booted, the Migration Assistant will spend some time surveying the system and ask you about what users, applications, and settings you want to move. Generally, you want to move all of these things. However, if you want to reinstall your applications from scratch, you should make sure Applications is not selected.
6. Long wait... you can go get a coffee in the next state (not really, but it feels like it) or do something else you enjoy that doesn't require your computer. Once it's done, you'll be ready to continue to the log in and your new system should feel very familiar.
7. Check your settings and make sure your WiFi network is connecting and that your email works. Those things are pretty crucial, but should be easy.
8. If you have Adobe products, Authorize them now, so that you can make sure the deauthorization worked before you erase the hard drive and make things very difficult for yourself.
9. Authorize your iTunes account if you have one, which will make sure you can play your video and music, and also verifies you haven't run out of machines that are licensed.

At this point, you're ready to go

Migration (Network)

If you have a MacBook Air (or you just like sitting around a long time), you can use the network to migrate your files. The process is very similar to above, but with a few changes, which we will detail here. When you reach the "Plug the Firewire cable in to both machines" step, follow these instructions:

1. Make sure your old machine is booted and can talk to the network
2. Answer the Migrate from Another Mac question with Use Network
3. Follow the instructions by making sure the software on that machine is up to date, starting Migration Assistant, and selecting To another Mac on the Select a Migration Method pane.
4. Once the computers discover each other, you will need to enter the passcode displayed on the new machine's monitor into the older machine.
5. Once the old machine is connected, the Migration Assistant will spend some time surveying the system and ask you about what users, applications, and settings you want to move. Generally, you want to move all of these things. However, if you want to reinstall your applications from scratch, you should make sure Applications is not selected.
6. Continue with the Long wait... step above

Preparing the old machine

Even if you are planning on giving the old machine to a trusted party, it's a good idea to erase all of the data. We'll go over the steps of preparing a machine to be given away or sold here.

There is really only one small choice to be made when following these instructions, and it centers around whether you will be installing a newer version of OS X than the one that came with the machine. Keep in mind the licensing issues mentioned above (mainly that you will be giving away the license to whatever operating systems go with the machine, which means the original DVDs and manual, as well as the right to use it). If you are, for example, running Leopard on the machine now, but bought it with Tiger and don't want to give away a copy of Leopard with it (or can't find your DVD, or used a Family Pack to upgrade that machine), then you will likely want to install the original version of OS X that came with the computer.

Once you've made this choice, you're ready to proceed to the next steps.

1. Make sure everything was thoroughly backed up before doing this, as there is no way back.
2. Insert the installation disk (either the original one that came with the machine or the new version of OS X that you will be transferring with the machine) and reboot
3. When you reach the initial installation page, select Disk Utility from the Tools menu. This will start the familiar Disk Utility program in the safe environment of the DVD.
4. Select the Hard Drive that is in the Computer (there should only be the Hard Drive and whatever DVD is in the removable media drive, so this shouldn't be hard).
5. Click on the Erase tab
6. Select Security Options... and in the follow-on window, select 7-Pass Erase. This is pretty darned secure (and the DOD considers it a standard for securely erasing media), so it should be more than sufficient for most needs. However, the 35-pass erase is available for the über-paranoid among us. Don't even think of using the first two options, they are way too risky... and if you're thinking "Oh, I'm giving this to my mom", you should consider that Mom can have her laptop stolen just as easily as anyone else, so you have no idea who will have access to your old data if you don't remove it sufficiently.
7. Click OK to go back to the Erase screen
8. Click Erase... and then thoroughly read the confirmation screen and follow the instructions.
9. If you thought you had a lot of time for coffee before, you can go to dinner or to sleep now, because this takes a really really long time.
10. Once this is done, you can quit Disk Utility and continue to the installation process.
11. For the installation of machines that I'm sending to other people for whom I'll have no further responsibility, I usually follow the instructions right up to the point it asks you to provision a new account. If you're going to be selling the machine, this is a good time to hit Command-Q and quit out of the installation process. You will be prompted by OS X telling you that it will have to shut down and complete the process later, and that's just fine. If you continue through here, you're just putting a lot of work in the new owner's lap.
12. However, for installation of machines that I'm sending to my parents (or any other machine that I'd be responsible to run remotely or to repair), I always install an administrative account for myself so that I can reset the password if somebody changes their password or otherwise confuses the machine. Then I let the installation process finish and add a user for whomever the machine is going to and configure information for them.

At this point, you're done. You've now succeeded in moving your old data to your new Mac, removing all data from your old Mac and installing a fresh OS. Now pack up the OS disks and the original installation media into whatever container you'll be using (perhaps you saved the original box) and deliver it to the new, happy, Mac owner.