We've had good luck with Mosquito Dunks, both in natural standing water sources and in dedicated "trap buckets" filled with water and vegetation, but you have to do it early in the season as the dunks only affect the larvae.

Last year I came across a water-filled stump that was teeming with larvae swimming around in it. Dropped a piece of a dunk in, came back 24 hours later and the water was clear.