biting & jumipng

Question
I have an 8 mo old spayed female lab mix.  She is a very good dog but jumps on us and bites very often especially with my 8 year old son.  We have tried holding her mouth closed but that seems to annoy her.  She was attacked by a pit at 10 weeks and suffered numerous injuries but is fine now.  Could the biting be a way to protect herself from another attack or is it just normal lab pup behavior that with age and consistency will diminish?  Other ideas to control biting & jumping?

Answer
Yelp when she bites and walk away when she either bites or jumps up.  Never give her any positive attention for either.  Every member of the family must be totally consistent about it.  Even a little petting once in a while will be enough for her to keep doing it.