In general, if I have a single one of a wind that is neither the round wind nor my position wind nor the dora, I’ll discard it without hesitation.
Failing that, if I have a single one of my position wind (unless it’s also the round wind), I’ll discard that without hesitation.
Lone dragons are next to go after that, then the round wind is on the bottom of the list.

Also, I’ll usually do most of my thinking during other players’ turns. By the time my turn comes around, I’ll usually have decided what I’ll discard in most cases. Usually I’ll have one tile that I’ll most likely discard, save for a few unlikely cases for the tile I draw. If I’m playing a game with a double-click/tap system (Jan Ryu Mon or Mahjong Fight Club, for examples), I’ll click/tap that tile once, then as soon as I verify that the tile I drew isn’t one of the special cases in which I’d discard something else, I click/tap the tile a second time to discard it.

During other players’ turns, I’ll also decide what tiles I’d chi or pon, and what tiles I’d pass on.