If you’re carrying out any sort of maintenance on the inside of you PC, you should have a specialist cleaning aerosol air duster. This is basically “air in a can” that comes with a direction nozzle for you to point and squeeze.

click to enlarge
These cans are great for clearing dust out of your PC, and can be used on your PSU. It is important however not to point the air into your power supply from outside the PC chassis however – when you switch your PC back on, the dust will just get sucked back into the PSU.
Effectively clean your PSU by disconnecting your PC from the mains, removing the side and lying the chassis on its side with the motherboard closest to your desk or bench. Spray your jet of concentrated air at the internal power supply fan – this will force the dust into the PSU and out of the external fan grille.
For extreme dust that has really caked onto your PSU, you’ll find it is easier to disconnect the power supply cables from the motherboard, drives and other devices, and remove the unit from the chassis. You can then use a vacuum cleaner hose attachment (the sort that doesn’t have a brush) to suck up the heavy duty dust to give your PSU a good clean out.
You can reduce dust build up in your power supply unit by maintaining a clean and healthy working environment, regularly cleaning your desk, vacuuming your floor and using an anti-static duster on your PC, monitor and peripherals.