Clean out the computer as often as is possible to keep dust from caking up inside it (you'll have to open the system to give it a good cleaning, a tank vacuum, can of comprssed air, or your own breath (you might want a bandana to cover your face) will work just fine).
Another idea would be putting it up off the floor, if it can sit on a desk it'll pick up less dust than it will shoved under the desk. You might consider new fans with air filters, but you'd have to remember to clean them often (weekly or so) or they'll fill up with dust and become worthless for cooling.
Setting up my desktop in an office where there is more dust than typical due to human traffic patterns, etc. What are some considerations and precautions that I need to keep in mind and practice.