Unless you need advice from a blue shirt working the floor or what you were buying is over $100, you can pay and check yourself out with the Apple Store app [1] and what they call their Easy Pay feature. I find this especially convenient when a store I go into is very crowded or I just don't want to wait. I grab what I need, scan it with the app, key in my password, and walk out usually within 5 minutes.

[1] http://itunes.apple.com/us/app/apple-store/id375380948?mt=8