What I do is (once a year for the big clean-up) take out the ram, fans, hard drives and any water cooling components ( have AIO radiators so I don't have to worry about water) vacuum them then blow them out (removing the fans from the radiator) then wipe carefully every thing down with a lint free cloth.
 
Make sure you keep the fans from spinning when you blow them out, especially with the GPU.
Same with the power supply and the fans with the cpu heat sink.
I use Q-tips with alcohol to clean the edges of the fans. I have a cat that likes to crawl all over the place so the fans like to suck in the oily hair and gunk them up. I use 91% rubbing alcohol to get the gunk off.  
 
I then reapply thermal paste and put it all back together.
 
Although it is a pain it can solve several problem you may not know you have or yet to have.
I have a 7 year old computer that the cpu was getting hot so I finally took off the heat sink and found out that the thermal paste had turned to dust!!!
 
So when you put back together clean the contacts as they do get fouled up and make sure put the wiring back snugly (had a ram chip that was giving me errors because of this and a hard drive that the cabling became loose). 

Leaf blower. 

This:
 
https://www.gearbest.com/brushes/pp_1620879.html
 
Attaches to most all normal vacuum cleaners

EyeDeeNo

Sajin
Leaf blower. 

+1
 
Electric or gas powered?
 
Edit: I try to clean out dust as often as I can once a month I'll take the side off (it's a not see-through panal) and at lest look what's doing. For an annually deep cleaning I will take the system outside to use an old toothbrush and can of compressed air or my own breath. The best is when I get face full dust from blowing into it.

Gas of course.  

and most importantly is to keep the room clean as possible, i get very little dust in mine. us a good air filter on the furnace/AC unit, it does wonders.

take your rig into the garage and use the oldladies hairdryer. keep an air purifier running in your office and you will be golden!

data vac duster hands down
https://www.bing.com/vide...3&&FORM=VRDGAR

Remove all, and vacuum.  I then use micro fiber cloths with believe it or not Meguires interior auto detail spray.  I put a bit of this onto the micro fiber cloth, wipe, and it pulls all the dirt away.  No residue. Easy to make it look new.  
 
Oh and never used a air compressor for things such as a nail gun, or one in a work shop.  Those have oil in the compressor and tank and well, you will not be happy.

Keep in mind to much airflow will hurt fans and can damage computer parts. The best thing is a small data vacuum since I have found that canned air isn't that good for long term.

No this is not true. If you want to stop dust from filling up your computer you need to remove it from your environment.. like live in a manufacturing clean room.
Dust filters only stop the big to medium particles but allow fine debris pass and I think that the small stuff is harder to get out of the case and I would rather have it mix/stick to the bigger stuff.
 
What is dust?
Mostly textile fibers from clothing, bedding and carpet, human hair and skin cells, outdoor soil and plant pollen.
The skin cells is the worst because it will have oil in it too and that's the stuff I want stuck to textile fibers.

All amazingly good advice hear. My personal preference is once a month, disconnect PC, take outside/garage and used compressed air. A decent air compressor can be had for $125 and has a verity of amazing uses. Don't waste your money on cans of compressed air......they simply don't do a good enough job. Be sure when using a compressor to hold fans in place when you are blowing them, learned my lesson there. Whatever a compressor doesn't get, get with a q-tip, microfiber rag, or toothbrush. The cleaner you keep your home & the less humans & the pets the better(I am screwed in that department ). What Rj posted @ post #6 looks like it would be a super awesome contraption that might be worth looking into.......

LOL, fire up the air compressor and blow it out, cleans up really nice and ready for overclocking and gaming :)

I do it using an air blower and a brush. Compressed air cans are damn expensive and are not much effective too.