Linda Cobb, "The Queen of Clean", recommends flipping on a seasonal schedule: side-to-side in spring and fall, and end-over-end in summer and winter. We found this illustration showing proper rotating and turning technique. The illustration seems to be missing instruction of how to manage while two crazy cats run ontop, over and under the mattress while you're trying to extend the life of your mattress, as we often have to manage, but it's a great visual reference nonetheless. How often do you flip/rotate your mattress in your bedroom?
Since getting our new mattress, we've been flipping (actually rotating 180 degrees, since it's a no-flip mattress) the bed every other month. Yeah, a little extreme, but we're freaked out about making the pricey investment last.