I work out with ben-wa balls on a daily basis (don't laugh). This is the single most beneficial exercise that I've found. I would recommend them for all musicians, actually, not just drummers, and also anyone who does any kind of repetitive tasks with their hands, like computer keyboarding. They keep the wrists and lower arms supple and stimulate all the nerve endings throughout the area. You will notice the effect the very first time you use them. You can find them in most Asian markets. The bigger the balls the better of course
, but you might want to start with a smaller set at first until you get the hang of it. Other than that, just keep your lower arms, wrists and hands stretched out at all times and aim for complete relaxation whenever you're playing. If you have a complete absence of tension in your body, there's little or no chance of injury. There's also a yoga move known as the crow which is good for stretching the wrists and lower arms. I've been drumming for over twenty years, injury free.
I do lift weights as well, but I don't find that it affects my drumming at all, for better or worse. The fastest, strongest drummers out there are the ones who are the most flexible and the most relaxed. It has nothing at all to do with brute power.
What kind of drumming do you do?