Some more random tips:
1) Go for square or star-shaped line rather than round. The sharp corners cut better.
2) For
gasoline, either use it all THAT DAY, or go non-ethanol. Them corn squeezins corrode small engine parts like no other!
https://www.youtube.com/watch?v=bEf9Fdvx_Sc
3) They make straight-shaft trimmers that can cut under fencelines easier and can be fitted with sawblades for removing tough brush. If that's not you, however, you can go with a cheaper curved-shaft.
4) Do you even need one? Ever since I got my scythe with brush blade I haven't needed my old trimmer. Very little maintenance and a much more pleasant
experience to use.
5) Always, always, ALWAYS wear eye protection and shoes! Okay, sounds pretty basic, but I was in the middle of something else and needed to clear some stuff up real fast. The goggles, I always keep hanging around the machine's D-handle, so I put those on, but didn't bother to go inside to put on shoes.
Everything went great until it decided to grab some chain link, crawl along it, and right over the top of my bare foot! Hurt like crazy and made a mess, but worse, there's a vain that runs right across the top of your foot... As it started swelling bigger and bigger, so did my eyes. Thankfully it never popped, because if it did, I'm sure a day in the emergency room would greatly outweigh any time saved by cutting corners!
Wear SHOES!