If you're talking Raining Blood-esque triplets, use your wrist and stay relaxed. Treat practicing triplets like practicing sweeping. Practice to a click, start slow and work upwards in speed and get it as fast as you can while you can still keep going for a long time, nice and tight, then push a little beyond it and practice that until you can do it solidly, and so on. Next time you pick up your guitar start slow again, but a little higher than you started last time, and push a little higher, and so on.
I'm personally aiming to be able to play triplet gallops as fast as the mid-section in Origin - Portal. Check that track on youtube and listen to how fast they're playing those little galloping triplet runs. If you can ever get to that speed and play comfortably, you're sorted. Haha.