When you launch iChat for the first time, you are prompted to add an existing account that you have set up (if you haven't set one up, you can create one from here as well). If you subscribe to .Mac, you would use your .Mac name (for example, joeshmoe@mac.com).

In some cases, it may be useful to switch back and forth between several accounts quickly, whether it's your significant other's account, or maybe a second account for yourself. Initially, you may look at using multiple iChat accounts as a challenge. That is until you remember that you are on a Mac, and things are typically easy to do on a Mac.

Once you have iChat configured for your first account, you will then go into the Accounts portion of iChat preferences to add the second (and third, and so on). To do this, you will simply click on the [+] button. From here, you will tell it the type of account that you have, and fill in the details. Below is an example:

After you have configured the second account, close out of the preferences. Form here, you will want to switch to the second account. To do that, simply go to iChat > Switch To > Account Name (see below):

iChat will quickly log you in as the other account, and you are set to go. To switch back, do the same thing. Obviously, you can do this as much as you need to.