Unless you already have a small air compressor (like the ones used for airbrushes), get yourself a can of spray air. Use something to stop the fan from spinning (the spray air could make it spin faster than it's meant to). Spray away! You may want to do this outdoors to let the dust-bunnies go free and avoid the dust getting in your eyes and up your nose.
You can use cotton wool buds on sticks ("Q-Tips") to get to awkward corners, in conjunction with some isopropanol or tepid water with a miniscule amount of washing-up liquid if there is any stubborn dirt build-up. Be sure it is has thoroughly dried out before switching on.
Remember to clean the PSU too, something non-metallic like a cocktail stick should reach the fan to stop it spinning.
Don't use a vacuum cleaner: a lot of static is created at the nozzle. This can be fatal to a computer.