This video is geared for puppies but all the principles still apply
http://www.youtube.com/user/tab289#p/u/18/ZKjk84OkzcI
The keys here will be re-directing your dog before it starts mouthing, a good off command, and lots of good play and training to tire them out.
Oh, and incidentally, Chihuahuas are statistically second (below Dachshunds) for dog bites and aggression.
