Airport Connection issue

System Preferences>Network>Advanced. Is the remember box checked?

You'll think I'm crazy for asking, but do you have an I-Phone in the same room? I tried for weeks to get my MacBookPro to stay connected then stumbled across a post where someone said the same problem was caused by the I-Phone's wifi interfering. The solution they suggested, which so far has worked like magic for me, was to reset the phone -- Settings, General, Reset, Reset Network Settings.
Literally the second i pushed that last button my AirPort card began to work perfectly. Good luck.

Sometimes my iPad or iPhone just don't see the network but I restart them. That was just something I wanted to share but do you have any other wireless devices such as your home phone? Disconnect all of them and see if your Mac can connect automatically. If it works then see if can change the frequency of the other devices or if you can't do that or you don't have a clue what I am on about use the Airport utility and find the option which will fix although you do get slower connection as a consequence

Couple of Apple technical bulletins:

http://support.apple.com/kb/TS1819

http://support.apple.com/kb/HT4014