Mt McKinley's rigging tutorial has some very good info on weight painting. I had books on it, but McKinley's tutorial explained it in PLAIN ENGLISH, and I was up & weight painting in no time! I HIGHLY recommend it, and you can download it right here!

Weight painting alone may not get rid of ALL bad deformations. Adding joints may help (ie a ribcage), but for arms & legs one effective method is to add blend shapes. Basically, you duplicate the offending geometry (ie the arm) and create a blend shape. You then set the blend shape slider to 100% and start pulling vertices on the duplicate arm until it looks the way you want it to. You can then integrate the blend shape into normal arm movements through set driven key or expressions.

Hope this helps!