It's recommended you flip your mattress four times a year, but it's difficult to remember to actually flip it on time. To help you remember, Martha Stewart recommends the same trick hotels use: add labels to the mattress with the month the mattress needs flipping.
To help you remember when to do it, you can attach a label to each end of the mattress. On one end create a label that says January right side up, and April upside down. On the other, create a label that says October right side up and July upside down. When you get to one of the labelled months, flip and rotate the mattress so the label for the current month is facing up at the foot of the bed. To attach the label, you can use a safety pin or just take a marker to your mattress.