Others have all given you good suggestions. My tip has to do with the habits of dragonfly's. You may have noticed this about dragonfly's already, but just in case you haven't.
When looking for Dragonfly's I will slowly try to get as close as my lens minimum focus will allow. If the dragonfly flies off I stay at the perch it flew from, and wait very still for it to return to the same perch. More times than not it will return and you will be ready for the shot. Good Luck