I encountered a strange issue where Mac OS X Lion stopped remembering a specific wireless network when my imac goes sleep or when i restart my imac. I googled some solutions about that but none of these worked. Than i tried to add my wifi network to Preferred Networks which is in System Preferences manually. And uh it worked :)
Solution is easy;
1-) Open your Network from System Preferences.
2-) Select Wi-Fi or AirPort from left tab.
3-) Open Advanced tab
4-) Check Remember networks this computer has joined.
5-) Remove your network in Preferred Networks if its exist.
6-) Click plus icon and add your network manually(Don’t forget to change Security to Your Wireless Security Encryption(Most of Wireless routers use WPA/WPA2 Personal or WPA) if your wifi has password protection.