Other than through product support, there are only two approaches I’ve found to address this problem. One is to reuse encounters from past games (adjusted for current circumstances, of course).
The other is to use similar encounters (some distance apart) in the same campaign, which gives you a chance to test out some elements the first time around. GMing experience also plays a role, as it gives you some baseline assumptions to work with.
Particularly in games like D&D, how do you go about creating solid, fun tactical encounters?