This is a great video by Jason Brown on how to prevent injuries in BJJ.
A great way to strengthen your feet and hands is to make a rice bucket. Get a bucket from Home Depot (http://www.homedepot.com/h_d1/N-5yc1vZ1xh7/R-202264761/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053) and fill it with 30 lbs of rice.
Stick one hand or foot in the rice and work it down. Then open and close your fingers/toes and move your hand/foot around in all directions. Make circles, spell the alphabet, make figure 8s, be creative. You can do this for time - start at 3 sets of 15 seconds (you will be amazed at how tiring this is) and work your way up to 3 sets of one minute. Try this twice a week and you won't regret it!