Some will say get a bigger battery, which is never a bad idea. But when push comes to shove, I just add lead - and as much as needed! I used to try to keep it light, but I have found that you are much better off adding a pound if that's what it takes than to be even slightly tail-heavy.

PS, and when i say a pound, I mean it. I have added over 20oz to some planes - And the still flew great