If I could comment on Grumpys idea of fixed head units. From my observations over time I have found they used these heads with thick line 1/the thick line lasts longer 2/you need a powerful unit to push thick line which is generally why you see contractors using them. They buy commercial machines, usually.
If it is adaptable, Kawasaki make a brilliant tap feed head. I keep mine well away from concrete edges and the like and it is still in perfect condition after 12 years. I was put onto them by a good friend who is a mowing business owner.
As others have said, keep the string up to them and don't go below 2.4mm, preferably 2.7mm if your machine can handle it, both the motor and the protective shroud cutting blade to keep even lengths.
I have found the Shindaiwa Speedfeed heads of good quality too.
