Here's a good article from T-Nation on improving your chin ups:
Keep Your Chin Up
Another interesting article about bodyweight exercises:
The MacGyver Workout.
My advise for both exercises would be to use a variety of hand postitions and, for pushups, feet postitions. For pushups: feet on a bench, feet flat, hands in a bench, hands in close, hands wide, palms in, palms out, etc. Just do as many as you can.
For pullups: wide grip, close grip, palms in, palms out, etc.