What are your best tips and tricks for keeping your computer from getting dusty?
The most important thing to do is to have some good filters on every fan that blows into the computer. In addition it can be very smart to have more fans that sucks in than blows out. This results in air going out of all the potentially unfiltered holes instead of in. Having a computer elevated a few inches, doesn't really help much. My brother had that, and his computer was full of dust when I checked it recently.
I have had this setup for years and the computer is pretty much dust free inside. Just remember to take out the filters and clean them once in a while for example by carefully vacuuming all the dust off of them. Should maybe be done 4 times a year. Maybe more, maybe less, all depending how much dust you have around :)
Personally I bought some filters that were specially made for 120mm fans (which I had), but I think that a friend of mine actually cut some pieces of a pair of nylon pantyhoses and I've been told it worked quite well. Haven't tried that myself though... so don't really have a clue :p
Keep the computers off the ground. Keep the room clean. Clean the computers, at least blowing the dust out, every six months. It's probably not worth going to more effort than this, since dust doesn't really do significant harm and periodic maintenance is a good idea for reasons other than dust.
On top of the fans/filters, one of the more interesting ideas I've heard is to put silica gel packets (like those often shipped with athletic shoes) in areas that you don't mind dust (the bottom, away from the boards).
The idea is that you can decrease the overall humidity in the case, so the dust sticks less to the components, and more to the packets. That makes it a lot easier to clean.
Basic steps to mitigate dust:
Whilst you can get filters, don't forget to replace them, because otherwise you may adversely affect the cooling capacity of your system, and end up toasting the unit.
You put a filter on the intake fan and about every six months you drag it outside and blow it clean anyway. Works perfectly. I have my computer on the floor and make no effort to keep dust away from it. Try not to use a vacuum cleaner if you can avoid it, since it causes static electricity to build up and can potentially destroy components, use a can of compressed air or a compressor instead.
As Rowland mentions, you should of course clean or replace the filter now and then, check it every month. There are special computer cases that are constructed with this in mind, such as the P182 from Antec.
Alternative cooling methods such as water cooling can eliminate nearly all dust buildup. This likely isn't viable in your situation, but it is still a solution nonetheless.
You can also try to sweep/vacuum the floor often.
There are dust filters you can place over the fans and any vent holes. So long as you are not overclocking the system, the room is well ventilated, and the fans are working well, this should not cause problems with system cooling.
Here's a link to an amazon.com search for computer case dust filters: http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=computer+case+dust+filter
Used dryer sheets work well as dust filters over the intake fans. They need to be used though new ones have too much of the "smell good" stuff on the to let air flow well. As the others have mentioned a good clean area and/or "real" intake filters work as well.
Elevating your computer just a few inches off the floor should solve the problem. If your house has a lot of cat dander, maybe setting the case on a desk would be a better idea.
I'll put in another plug for filters. Many custom cases, at least, come with a filter or a slot for a filter in front of the primary intake fan, but you can make your own. I've opened up all the unused drive bays and taped thick filter fabric over them, so not much dust accumulates inside.
Getting good dust filter grills and aluminum mesh or steel mesh filter , is a lot easier than it was before. http://www.xtremesystems.org/forums/showthread.php?209801-Some-Wire-Mesh-Filter-Fan-Grill-Testing... . Specifically not this type http://www.performance-pcs.com/catalog/index.php?main_page=popup_image_scroll&products_id=2334&image_scroll_id=0 a filter Mesh not a "Grill"
I have always applied filtering methods in my computers, because of how many wires I have connected to them, it is difficult to just Pull it and take it outside to use the compressor on it. Canned air, and vaccumes are almost useless.
When filtered the flow is reduced, there is not a filter method to date that does not reduce the air flow some, and the filters have to be properly maintanced. Because of that it is nice to have them easily accessable and not stuffed in the case. . At first the temperatures of the computer can be higher , or require more fan speed in thermal control situations. In the Long term (years) the filtered computer Maintains its temperature much better, because of the lack of internal dust buildup. Eventually the computer still has to be cleaned anyway, about 1/4 as often.
Some of the "Foams" that we got in fan grill items broke down over time (about 4-5 years). Plastic meshes are terrible for air flow, cloth type meshes lasted longer but are not as free flowing as foams. The newest aluminum and steel meshes they are comming out with are less restriction , and longer lasting. Any Metal meshes should be mostly Air, not metal. My favorite DIY is the foam from the top of a Shop-Vac , the filter foam that is used when doing wet work. it allows for very free flow, stops all large dust particles, lasts time, cleans easily, and comes in a size that can have some good coverages, and available locally.
Store your computer away from cooling or heating air vents. If the air vents in your home or office are not cleaned regularly, they can release dust into your computer if they are nearby.