If you disturb a perched dragonfly it will often return to the same perch, or another close by, so set yourself up and sit still for a few minutes. This is good for darters, chasers and skimmers.
Hawker dragonflies spend long periods patrolling on the wing without perching, but they tend to follow the same circuit so after observing them a while you can predict where they are likely to fly. You might then have the chance to photograph one of them in flight. This is immensely frustrating as it is possible but difficult, and requires not only a rapid autofocusing system on your camera (ideally one that will track movement away from the initial focus point) but also developing the skill of finding a subject quickly with a telephoto lens. They seem to have rest periods, but often at different times of day from when they are patrolling (late afternoon or early evening on a sunny day is good), and often rest in trees fairly high up a little away from the water.
Dragonflies are also much more approachable when they are eating, and having a mouthful of prey can make for a more interesting picture. I photographed one dragonfly in Thailand eating a honeybee which also had a small fly buzzing around trying to steal some, AND a mite biting its head. On the other hand when they are mating they are still easily spooked and will fly off together even if the male is in the process of transferring the sperm package to the female.
You may also catch them in the process of laying eggs.
I find that a close-focusing telephoto lens is better than a macro for dragonflies. With my Canon system the 100-400mm f/4.5-5.6 L IS II is good and focuses reasonably close as well as having a remarkably good image stabilisation system (I bought it as a jack-of-all-trades to take on holiday and it has proven to be a very capable all-round wildlife lens). I also have the Canon 180mm f.3,5L macro, which is good for dragonflies and very sharp, but no image stabiliser, and the Canon 100mm f/2.8 L IS macro, which is a good insect lens all round, though probably better for smaller butterflies, beetles, bugs etc. rather than dragonflies. Health problems have made it difficult for me to manage a heavy professional camera, however, so I also have an Olympus micro 4/3 system and the 300mm f/4 lens is good for dragonflies, birds and many other things, being light enough to carry and hand-hold and yet giving the same magnification as a 600mm f/4 on a full-frame camera (it doesn’t have that razor sharpness of the Canon super-telephotos, however).
I generally don’t use flash for dragonflies as they usually appear on sunny days when there is plenty of available light. A ring flash on a macro lens is good for smaller insects, but gives rather flat lighting when you are far enough away for dragonflies.