Place a thin rubber mat or towel underneath the mattress. This helps provide extra friction so your own body movements won't cause the mattress to slide off the frame.
Adjust the sleep numbers on the two sides of the bed so they are closer together. If one side is very firm while the other is very soft, you are more likely to slide toward the center of the bed where the central support is. Equalizing the two sides a bit more will help negate this problem.
Contact Select Comfort customer support at 800-472-7185 if the bed is still giving you problems. It could be malfunctioning or may be assembled incorrectly and Select Comfort can check your warranty and help repair it.