Resolved my Wifi issues after upgrading to 10.6.8

Updated 01 Sep 2011: Added in the command to change the Airport default behaviour
Updated 02 Nov 2011: Added the removal of com.apple.alf.plist 
Updated 04 Nov 2011: Added updating of DNS entry
 
Recently I had a lot of Wifi issues with my Mac Mini when I updated my Snow Leopard operating system  to 10.6.8. I was unable to get a reliable Wifi connection and I had no wired connection as I wanted a "no-wire" home. I tried many solutions such as recreating my Wifi connection, changing channels and even changing the encryption type but all didn't work.

After researching for a few weeks, I finally found the magic bullet to resolve the Wifi issue. For my situation, it was the DNS cache issue. To resolve the Wifi issue, just do the following:

1. Launch Terminal
2. Run sudo /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport en1 prefs JoinMode=Preferred JoinModeFallback=KeepLooking
3. Type in your current account password (assuming you're in an administrator account)
4. Run dscacheutil -flushcache
5. Run rm /Volumes/Macintosh\ HD/Library/Preferences/com.apple.alf.plist 
6. Add the current DNS IP address of your Mac under Network > Advanced

That's it. Point 2 will help to set your Wireless to by default join your preferred networks and if it is unable to find your preferred networks, it will continue looking for it. Point 4 will in a way help you reset your Wifi settings (DNS). The file in point 5 somehow got corrupted I think and removing it resolved the issue.

My wifi speed somehow became faster (but still not as good as before) after adding the DNS IP address in the Network settings. I find it weird but I won't question speedtest. Take note that you should add it in even though your router or modem is already giving you a DNS IP address which you can see in a light shade of grey in the network settings. Add it in anyway.

This series of steps resolved my Wifi issue. Whew...