Mac: Fixing auto connection to WiFi

I've had a hell of a time trying to figure out why my Macbook wouldn't auto-connect to my preferred networks and Googling over and over again didn't help. Basically, up pops the "none of your preferred networks are available" window. But finally, this forum post actually worked so I figured I'd repost it here. Since it's done so succinctly, rather than rewrite it, I'd just reprint the very, very helpful instructions here:

I have been searching high and low for a fix to this and none have worked…until now!

I spotted a fix to this problem that strangely centered on the specific location of the “System Preferences.app”, if other than the standard location of the root of the “Applications” folder. I had in fact moved mine and would NEVER have thought it related to this bug. Hmmm!

If you have moved the System Preferences application (not alias, actual app) to any other location for any reason, your newly joined preferred network will NOT be properly remembered and will NEVER be properly rejoined. Apparently one of the multi-step system procedures involved in properly saving a preferred network assumes the location of System Preferences to be the standard location. Period.

FIX:
1) Move “System Preferences.app” back to “Applications\”.
2) Delete all instances of “Preferred Networks” in “Network Preferences”.
3) Delete all “Airport network password” instances in “Keychain Access”.
4) Restart your Mac.
5) Add/Join applicable preferred network using the Airport pulldown menu or in Network Preferences (as if for the first time). Use “Advanced” in “Network Preferences” to confirm the existence of your preferred network and the correct settings. Use “Keychain Access” to confirm the existence of the applicable “Airport network password” instance and the correct settings.
6) Restart your Mac. The OS should now find your preferred network and join it on its own. Good Luck!

NOTE: Simply moving back System Preferences will NOT suffice. Deleting and re-adding/joining the preferred network is a must after the app has been moved.

Hopefully that solves your problem too. I'll add in my two cents that I followed these instructions and it worked beautifully. As an added bonus, I think my Macbook boots up much quicker too.