In addition to JMH's excellent suggestions above, a word of caution just in case you're not aware of this fact: If you're going to use a "vacuum" to suck the dust out of a system, be very, very careful that it's a special battery-powered handheld vacuum especially designed for computers. Do NOT use a regular, plugged-in vacuum cleaner, as that could seriously damage your system!!!
Having said that, if you are using a computer vacuum, you can even tape a straw to the end of your vacuum - it will drastically increase the sucking power, and give you more control/detail for the tight spaces.
Be sure, of course, to vacuum the vents. Get the wand and put it on each vent and create a seal with your hand around it and just suck the heck out of it. Also, block the other exhaust vent to create a vacuum inside to make sure you get everything out of there.
If you're a computer guru and have the ability to remove the back to actually get inside and clean it, even better.