I think using weights would probably help (e.g., leg press) and/or working on your speed while doing the kick (e.g., wearing some ankle weights while doing the kick -- but don't wear too heavy of ankle weights as you can injure your joints if you over do it). I have the opposite problem. I have plenty of power in my kicks, but I am really inflexible and can NOT get a side kick up very high at all. So, if you (or anyone else) has any good recommendations as to what kinds of stretches I can do to get my side kicks higher, I'd really appreciate it.