This is one of the problems I ran into with the iMac line.
Living in a dusty/dry area, dust in your home is unavoidable, unless you clean every time you open the door.
This is also why I moved to the Pro line after losing my iMac to dust twice. (Both covered by AppleCare at the time).
To the average user, the iMac is not serviceable. To clean it, you wipe off the vents at the bottom, and the exhaust port on the back. As suggested, you can power down your iMac and shoot short bursts of canned air into the vents. However this will only do so much. The fans will remain caked with dust.
Sadly, the only way is to disassemble the unit, which takes lots of precautions. Static and the fact you only have 2 hands hinder in this process. Search for disassembly of an iMac and you will find lots of walk through, however if you are still under warranty, take it to the Apple store.
It will take 4 hands to safely disassemble the unit for cleaning, as well as a special torx head screwdriver bit. The risks greatly outweigh the advantages of dusting an iMac unless you are 100% certain, that your area will have enough dust to need cleaning.
It is unfortunate that you cannot easily clean the boards, however to add that kind of capability disrupts the sleek design. As an Apple consumer for quite some time, when it comes to their outward case design, it is form over function.