If you use an air compressor, be carefull. They are great for removing dirt and dust, but they also go a great job of forcing dirt/dust into bearings.
+1 on cleaning it throughout the day, simple green sprayed on a towel works great.
I tear my cars completely down after each race day, maybe not the diffs. I clean everything by hand, re-do the shocks, check nearly every part for wear, damage and dirt, make sure each part moves freely. Haven't had a mechanical failure in 15+ years!