I found this article about making a Mosquito Bucket of Doom. 

1) Fill a bucket with water and put it outside.

2) Add a handful of grass to make carbon dioxide which will attract more mosquitoes.

3) There are two options to kill the resulting larvae.

-- Dump the bucket weekly and refresh the trap with new water and grass.

-- Add a mosquito dunk, which lasts about a month.  Replace whenever it sinks or dissolves.


Also, make sure there are no other pockets of still water to attract mosquitoes elsewhere in your yard.  Every bit that you can find and remove is one less mosquito nest.