I have tried String leeches, string bats, and cat whiskers on the string. Cat whiskers proved to work the best, last longest, and are pretty cheap. The other thing to think about is an STS device. There are a couple guys that make these and sell them over the web.I have an STS that I got at a pro shop and it works very well but costs $40 -$50. The internet ones are cheaper put not as adjustable.

If you don't already have them start with the cat whiskers and see what that does for you.