My MacBook Pro loses its connection to my WiFi router
Mac Wireless &amp; Airport Connection Problem Troubleshooting: The Basics
* Turn Airport on &amp; off – You can do this via the Airport menu bar or from the Network Preferences. This is the first thing you should try when troubleshooting Mac wireless problems.


Reset your router – This is the second thing you should try doing. You can fix a surprising amount of wireless problems just by resetting the airport/router. All you need to do is turn the thing off for a few seconds and turn it back on.
Reset your Cable/DSL modem – You’ll usually want to reset this in combination with your wireless router. Reset this first so the DHCP information will be pulled to the wireless router properly.
Change Wireless Channels – sometimes your router’s wireless broadcast channel will interfere with a neighbors, be sure you have your router set to a unique channel. Even if it’s a weak signal there can still be interference.
Make sure Wireless/Airport card software &amp; firmware is up to date – This is usually done just by going to the Software Update menu, if there are any updates available for your Mac or Airport, install them.


Mac Wireless Troubleshooting: Intermediate
* Change wireless security protocol – You shouldn’t be using WEP anyway for security reasons, but sometimes changing from WEP to WPA/WPA2 or WPA to WPA2 can resolve wireless connection difficulties.


Make sure router firmware is up to date – Check your router manufacturers website for firmware updates, if there are any available, install them.
Delete and recreate connection – Try deleting and recreating/reestablishing the wireless connection, sometimes a setting can be corrupted and this may fix it.
Create a new Network Location – Similar to the above suggestion, try creating a new and different wireless network location to see if it resolves the connection problems.
Change DHCP auto settings to manual – sometimes there is a problem with the DHCP server, and if you manually set an IP address on the network you can be fine. Remember to set the IP to a high number so it wouldn’t interfere with other DHCP machines. As long as you have the subnet mask, router, and DNS settings configured manually as well, this shouldn’t be a problem.
Disable “Wireless G/N/B only” mode – Sometimes a setting is selected that only broadcasts your wireless signal in Wireless B, G, or N mode (depending on the routers abilities). If this is set, try disabling it.
Flush the DNS cache – Launch the Terminal and enter the following command onto one full line within the Terminal:

dscacheutil -flushcache



Mac Wireless Connection Problem Troubleshooting: Advanced
* Zap the PRAM – Reboot your Mac and hold Command+Option+P+R during restart until you hear another chime, let the Mac boot as usual.


Delete Wireless Config files – Delete com.apple.internetconfigpriv.plist and com.apple.internetconfig.plist files from ~/Library/Preferences and reboot
Trash your home directories SystemConfiguration – Remove all files within ~/Library/Preferences/SystemConfiguration/ and then reboot your Mac.
Reset your Mac’s System Management Controller (SMC) – For MacBook and MacBook Pro’s: Shutdown the MacBook/Pro, remove the battery, disconnect the power, hold the Power Key for 15 seconds. Replace the battery, reconnect power, and zap the PRAM and wait for 2 chimes before letting the keys go. Let boot as usual.


source: osxdaily.com
http://osxdaily.com/2009/12/22/mac-wireless-problems-guide-to-troubleshooting-airport-wireless-problems-on-your-mac/