I have given this trick away before...I love it.

Place the knee warmer's elastic band directly underneath the bottom elastic band of your bibs. Grab both bands between your thumb and fingers and "roll" them 1 time (effectively rolling the bib elastic under). This creates an interlock between the two garmets. They will never move, but are still easily removable when needed.

BTW - I learned this in little league as a way of keeping those stirup things in place when dressing in uniform.