I go out of my way to raise people nearby. I drop heals on someone if I see they're getting wrecked by a mob, and stay there until they finish it off.

Currently I'm farming Longstop for Battlemage gear, each and every tank I've queued with so far was new to the instance and I always explain the mechanics of each fight for them beforehand. It helps a lot to explain something to a new tank instead of just getting mad and leaving.

I answer questions in /sh as often as possible and try to offer suggestions if there's something better someone could be doing for what they need.

I tell stupid jokes in /sh. Not really paying it forward, but hey, everyone loves a good laugh now and then!