How can I have a pond but prevent mosquitos from nesting there?

In addition to introducing predators like fish and dragonflies and moving water, many folks use Rabon blocks which are fed to livestock to prevent flies and mosquitoes from breeding in the animal waste. Rabon (tetrachlorvinphos) blocks are nontoxic to animals but cause the larva to not develop and thus interrupts the life cycle of mosquitoes and flies. You can break the Rabon up and sprinkle it in the water to help prevent mosquitoes from breeding.

Rabon can be purchased at farm supply stores or via Amazon.

You may want to take a look at Mosquito Dunks:

Kills mosquito larvae for 30 days! Mosquito Dunks have been used by professionals for more than a decade and have proven their value in destroying mosquitoes - by killing the larvae before they mature into biting adults.

Made with Bt-israelensis (Bt-i), a highly specific biological pesticide, this product represent a major breakthrough for people and the environment when compared with the traditional toxic chemical approach to mosquito control. Will not harm people, pets, wildlife or fish. To learn more about Bt-i, click here for reference information from the Environmental Protection Agency.

Easy To Use:
Once a month place wherever water accumulates around your home - you'll be rid of mosquitoes for the whole season. Each Dunk will effectively treat up to 100 square feet of surface water regardless of depth. They may also be broken in half to treat smaller areas. Great for use in water gardens, flower pots, bird baths, rain gutters and decorative ponds!

product image http://cache.planetnatural.com/planetnatural/images/large/mosquito-dunks-large.jpg

I know this is late, but it may be of help for others.

Get some Mosquitofish. We were able to go to a resource in our County (don't remember which one) and they gave them to us for free. Started with 6 and now we have too many to count. The good thing is that they are self-regulating as far as population goes.

If you cannot find a county resource to get them from, check around for others who have ponds. Most folks will be happy to seed new pond owners with Mosquitofish.

Make sides of the pond as vertical as possible. It prevents mosquitos from nesting.