Last time I had to do it, I thought for a minute how to make sure I didn't mess up. I came up with:
On each of the 4 possible mattress positions, use a Sharpie to write a number 1 through 4. When it's time to flip, add one to the current side, and make that side be up. After 4, go back to 1.Ta-da.
This post suggest one way to do it based on time of year, but I think I'd forget what season meant what. They suggest flipping every 3 months, which seems like a good interval.
American Scientist Online seems to have the same system as mine, but it's buried in complex math, and they count from o to 3, so my system is obviously better.