It's simple: get rid of all the sub-forums people don't visit. If you're concerned about losing content just grab some of the sub-forums you want to get rid of and turn into a main archives sub-forum. Combine that with merging or incorporating some of the sub-forums into just one, at least for some of them, and you'll make up for a cleaner forum. When people have a lot of choice, they tend to get distracted, and most people just go on about posting on the main sub-forum on top of the page anyway. You want to guide them into just enough places they can post their content so you have people contributing more in a specific way.