To begin, open System Preferences and choose Users & Groups (formerly the Accounts pane, which has been given a new moniker for Lion). First, you’ll need to click the lock in the lower-left corner and type in your administrator password to make changes. You still remember that for now, right? Then choose your account from the list on the left and click the “Set” button to assign an Apple ID to that account. After you’ve done that, you’ll be allowed to toggle the “Allow user to reset password using Apple ID” checkbox on.
So. Many. Arrows.
Now let’s assume for a moment that you’ve lost your account password. With Lion, all you’ll need to do is to go to the login window (choose Apple Menu > Log Out [username] if your machine is set to automatically log you in). Once there, attempt to enter your password a few times, and eventually you’ll get a dialog box that will ask you if you’d like to use your Apple ID to reset your password. Funnily enough, Apple claims you’ll need to enter an incorrect password three times. On my machine, it’s consistently four. The horror.
To continue, click the arrow on that notification box and type in your Apple ID to reset your forgotten data.
One caveat, though: If you reset your password this way, you won’t be able to access your Keychain, which holds information like your network passwords and Apple Mail account logins. It will be off limits to you until you remember the old account password you’d used, and if you never do, then the data is as lost to you as—well, as something REALLY lost. Like my-ability-to-come-up-with-a-funny-closing-paragraph lost. So be careful.
And if you’re not sure if you currently have an Apple ID, if you’d like to change the password for it, or if you want to create one, you can visit Apple’s page for doing everything I just mentioned.