How do you make itunes stop asking for a password just to download every time an app is purchaced?

On the computer:

Underneath the passWord field, there should be a "Remember passWord" box. Click that, and it should stop asking for a passWord.

On an iDevice:

Your iDevice will remember your passWord for 15 minutes since the last time you entered it. You can purchase as many apps/music/books as you want in those 15 minutes, but you must re-enter the passWord once those 15 minutes are up.