I've always used new 1"-1.5" paint brushes with bristles as stiff as I can get. An old toothbrush works well, too, but brushes cover more area. Even using paint brushes, keep the toothbrush handy for hard-to-get spots. I use the same "equipment" and techniques for CPU cooler fans, too. You'd be surprised at how little dust and gunk it takes to noticeably increase the noise a fan makes. Depending on the general environment in which you live, once a month should suffice. I lived in the US desert southwest for years and cleaned everything 1-2 times a month. Here right on the Atlantic coast in Florida, I find I only have to do a thorough cleaning every 5-6 months. Just look on physical PC cleanup as a labor of love.