I've vacuumed computers out many times. I'll use both the soft-bristle dusting brush and, for things it can't reach, the narrow crevice tool that concentates air flow in a smaller path. In addition, I'll also use a small artist's paint brush (or a Q-Tip swab) by hand to get into tight spaces to loosen dust, holding the vacuum close by to suck it away as it is shaken loose. I'm not shy about dismantling things, so I'll usually open the PSU and clean it out, too. However, I recognize that usually means breaking a seal and voiding the warranty.
I will note that I do not live in a very dry climate, so static charges are not usually a problem. However, I certainly would not do this in a carpeted room!
Three major rules:
ALWAYS disconnect the machine from power, and WAIT 10-15 minutes in case there are residual charges in capacitors before starting. I disconnect EVERYTHING so there's no chance of a circuit through some peripheral device.
GROUND YOURSELF FREQUENTLY, just as if you were doing any other work inside the case, to ensure you get rid of any static charge build-up.
BE GENTLE!