There are a number of ways you can try and control mosquitoes.I've listed them in order of effectiveness
Prevent them having a breeding ground and/or kill existing larvae.
Basically most still water is perfect for mosquito larvae. We had some breeding in an old barrel full of rain and fixed the problem with a bit of washing up liquid; we noticed a huge difference within a couple of days. Bleach would also work.
If you don't want to put contaminants in the water you may be able to get them with a fine net.
Try and encourage mosquito predators
Bats, tadpoles, dragonflies/damselflies and some small fish will all eat them as either larvae or adults. Of these, dragon/damselflies/frogs/toads and their young will probably do the most damage to the local mozzie population.
This will probably take a lot of work so unless you already have a small pond or somewhere to shelter bats it's probably not viable.
Plant strong smelling mosquito repellent plant species.
The citronella geranium is probably the most well known one but there are a lot of plants that will help, particulary as mosquitoes find you by smell so having a generally fragrant garden may be a big help.
There is a list here that may help.
As an aside I've found a product by Avon called skin-so-soft to be particularly effective at repelling mosquitoes (as well as good for your skin).
Addition: Apparently different types of mosquitoes are affected differently by various smells so some plants commonly purported to deter mosquitoes may not necessarily be effective. This study suggests that citronella may not even work at all.