Using a can of compressed air is the best method. Cans of compressed air for cleaning computers can be obtained at most department stores or computer specialty shops.
The first step is to power down the system and open your case. As the computer is constantly circulating air (and dust), you may find that quite a mess is created when dusting out the system. Relocate the system to an appropriate area for cleaning.
It is not necessary to touch any components while dusting out the system. Take care to keep the can of compressed air upright, as tilting it may result in expelling the propellant. Start at the top of the system, blowing dust off of the components and working down to the bottom of the system. Fans and heatsinks tend to trap dust and often require extra attention.