First try the
Reset Your PRAM to see if that helps. Then if it doesn't you will have to get geeky and fix the Airport drivers. Follow these steps to reset the Airport drivers:
1. Open System Preferences->Network pane and highlight the Airport card.
2. Then in the right hand pane disconnect the Airport card from any wireless connections.
3. Turn off the Airport card!
4. Navigate to the folder/YourHardDrive /Library/System Configuration/ and delete the preferences: com.apple.airport.agent.000a95b77e90.plist & com.apple.airport.agent.37C8FC8B-0990-516C-BBD4-E9AFB9680E04.plist. Note these are what I have in 10.6.5 so yours might be different. Just get all the say Airport in them.
5. Reboot Immediately!!!! This is import to rebuild these files.
6. Go back to System Preferences->Network and highlight the Airport card. Then rejoin your wireless network.
7. Hopefully this will reset the Airport card.
Now the non-geeky way are in these steps:
1. Open System Preferences->Network and high light the Airport card.
2. Turn off the Airport card!
3. In the left hand of the Network pane, at the and click on - button and delete the Airport card.
4. Don't panic at the previous step!
5. Quit System Preferences and IMMEDIATELY REBOOT!!!
6. Upon the restart go back into System Preferences->Network pane and click on the + button and re-add the Airport card! Then in the right hand of the pane click on the 'Apply' button to save your adding of the Airport card.
7. Then put in your wireless network settings and try to join your wireless network and make sure you save those settings.
8. Reboot again to make sure you saved everything for new settings, for future reboots.
Lastly try all the steps in the blog post
How I fixed my dropping wireless Airport connection problem in Snow Leopard. If you tried all these steps and nothing happens then try a new wireless router and make a new test user account on your Mac. If the Aiport has problem in the test user account then you mostly likely have a hardware problem.