I've had decent success with the head edger in the past -
http://www.headedger.com/You will need to spend some time getting to grips with it before you start trying to trim heads in high-profile areas because despite what the informational video says, the unit will bounce. I have found that tilting the weedeater to the left and easing it down over the head is the best way to trim without bounce.
Also the teeth will last far longer if you are able to trim heads on a regular basis.