After years of being head brushcutter operator I have learnt a few things about using trimmers and brush cutters al day long:
Dress the part! You must have a good face guard and be fully covered up. However tempting it is to wear a t-shirt forget it or this will be the day that bits of dirt or stones fly up and cut your arms.
Wellies or real work-bots are the best footwear for using a trimmer. Anything with lace eyelets will get full of seed heads in no time.
All exposed skin needs plenty of insect repellent. Insects seem to appear from everywhere as you get cutting down the weeds.
Clean the air filter regularly (yes really as often as it says in the manual) or you will regret it, after the machine fails to start on the tenth attempt!
Remember to take the fuel can with you as you work, since you will always run out of gas at the farthest point from where you left it.
Gas brushcutters vibrate a lot! True, any good one (like my Husqvarna brushcutter) will now come with some kind of vibration dampening system. But if you use them for hours on end do pay attention to your hands. If they are starting to feel numb, stop working and go and have a cuppa.
Gas brushcutters are noisy too. You might need to take this into account if you live somewhere with strict noise regulations. And, even if you don’t you should consider your neighbours; let them know you’re planning on spoiling the peace for a few hours. Ear protectors might be a good idea for some. I need to be able to hear what’s going on around me (you never know when someone will sneak up on you ) so don’t use ear protectors, but you might be less nervous than I and find them useful!
If using a metal blade do take care with stones. Sparks are not uncommon if you hit a flinty rock with a metal blade. That’s fine in damp conditions but not so great in the middle of a drought surrounded by over-grown dried grass.
Trimmer heads for nylon line may need replacing over time (or at least the metal inside changing). It is possible to find solid metal fixed trimmer heads. These are great if you have stony ground that damages plastic models. There is some good troubleshooting information here.