For trimming face, feet, and booty, I use the Wahl Peanut. It is sold in most Beauty Supply places. Here is an example:
Wahl Peanut Clipper from Discount Beauty Products and Beauty Supply It is really small and light weight, very easy to use and comes with comb atttachments so you could use it for bigger jobs too. It was recommended to me by Mardelin who grooms and breeds her own Yorkies.