When asked if you are going to migrate your stuff from another Mac or partition, say no, and proceed to enter your user information -- make sure that your shortname is the same as on your other Mac or existing install. When you finish that, create a temporary account via System Preferences » Accounts.
Next run Software Update to bring the new Mac or install up to date. Run it again until it says there are no more updates, as some updates rely on others being installed before they will be available. Then log out of your account and into the temporary one. Now you are ready to migrate your original user and applications, and stuff from your old Mac or drive. Migration Assistant is located in /Applications » Utilities. Take the family out to dinner or a movie while your files are copied to the new Mac.
The reasons that you should do things this way, is to 1) make sure that your new Mac doesn't have older system stuff that may be confused by newer things in your existing world, 2) to allow you to move your user account over the new user account on the new Mac, and 3) it may take a couple of hours to migrate, so this is a free moment for family time.
When you're finished, there will be a Deleted Users folder in the Users directory containing the first account you created when you booted your new Mac or fresh install -- this can safely be deleted. You can also remove the temporary account in the Accounts preference pane.
[robg adds: I'm sure there are other successful migration strategies. Personally, I'm brutal about it: with each major release of OS X, or new hardware in the house, I start over. Format the drive, install OS X, and then manually add things back in. It's not that I don't trust Migration Assistant, but rather that this method forces me to do some serious housecleaning each time I upgrade.]

