Totally normal puppy behaviour

Find something else for her to play with - if she's nipping your trousers stop (chances are SHE'LL stop when you stop moving and therefore become less exciting) and give her something else to play with, maybe a nice soft little tuggy you can both have a game with.

Teach her a "drop it" cue.

Read "The Bite Stops Here" (Google for it) - very useful article.

This is such a common "problem" that I really think there should be some sticky threads on here about it - unless I've missed them?