Follow Buddy's advice. One additional excercise I also like to use. Take your heavier bow, bring to full draw as rapidly as possible, without jerking or yanking, then slowly let the bow back down. As slowly as is comfortable. a count of 10 is good, a 20 count is better if you can let down that slowly. The negative resistance will build strength faster than shooting because it is different than how you use your muscles to shoot.
I also really like using dumbell rows for maintaining and increasing weight. Do the reps very slowly and as with the excercise above, never forget that the negative resistance, lowering the weight, is just as beneficial as lifting the weight. Perhaps more so.
Hope this helps.