vacuum cleaner, but a lot of people will stay away from that for some static reason

. Most will suggest compressed air, you can find them in just about any electronics store, wouldn't use your air compressor in the garage unless you know for a fact it has a dryer and it's functioning properly. I mean if you really want to get it done take it out of the case, remove the fans so you can wipe them down also and then use a Vacuum cleaner (I mean who wants to blow dust everywhere in there house {guess outside is an answer}).
I wouldn't use any liquids, you will inevitably leave some moisture there which will start the whole dirt attraction

process even faster.
If you do use the vacuum cleaner method, be sure to ground yourself before touching any of the internal PC components. I let my PC run while I vacuum out the rads, never had any issues, and that 6.5hp shop-vac can clean a rad out pretty quick.