Does your iTunes program keep 'forgetting' your password. If so, this fix may work for you. Basically, go into your iTunes account via the program (yes, you'll have to put in your password) and click "Reset Warnings" down at the bottom. You can also try Deauthorizing and then Reauthorizing your computer via iTunes, but this didn't work for me.
What seems to have worked for me (in addition to resetting warnings) is to delete the SC Info folder as detailed below.
After iTunes 9.0.1 was released, a number of users were annoyed that the check mark button for saving their password no longer worked. As a result, each purchase or app-upgrade triggers iTunes to prompt for a password.[From Fix for iTunes 9.0.1 refusing to save your store password | iPhone Atlas - CNET Reviews]
Quit iTunes and any other open applications until only Finder is open.
Choose Go > Go to Folder.
Type /Users/Shared/SC Info and click Go.
If Finder is not already set to column view, set it by choosing View > as Columns.
Select the SC Info folder and move it to the Trash by choosing File > Move to Trash.
Restart the computer and open iTunes.
Authorize your computer to play your purchases. Note: If you have multiple accounts you will need to authorize your computer for each account. For more information on how to authorize your computer, see this article.[From iTunes repeatedly prompts to authorize computer to play iTunes Store purchases]