i use a dust vac once a week and it works BEAUtifully. we got two at my job and we get some where theres more dust just in the psu then there is in my room and i havent dusted in about a year
may sound dumb, but make sure all the fan screws are tight too, if they wernt screwed in perfectly, the slight vibrations of the fan spinning can cause the screws to slightly untighten.