If you are running Windows 2000/XP and AIM 4.8 then you can download the Instant Idler program to go "idle" and "active" with a click of a button.  Otherwise, you will have to:

To make it so you always appear idle you need to download this file:

Always Idle DLL: idlemon.dll

Now close AIM completely and then copy that file over the existing idlemon.dll file that is in your AIM95 directory.  Most likely your AIM95 directory is located at "C:/Program Files/AIM95".  Now restart AIM, and when you log back on, your name will always appear idle.

To revert back to a non permanent idle state, you must download the original idlemon.dll file and copy it back into your AIM95 directory:

Original DLL: idlemon.dll