what worked for me was first doing a software update download and then following these steps:
1. attempt a Restart (for me it failed due to Finder running some process)
2. go to Activity Moniter (under applications -> utility) and force quit Finder by first double-clicking the Finder process to open a window
3. on that window, click "Quit", which should give an option of "Force Quit", so click on that
then a lot of stuff starting freezing and going blank, and the screen was all grey except for a little loading icon on the bottom, but eventually my MBA restarted and I was able to log in back into my account. I've been freeze-free so far...
my problem possibly lied in some error in the Finder function. my desktop was frozen for months but i never really paid attention to it until i started having these super frequent freeze problems. so the Finder reboot and system update/restart probably set everything back to its usual function