The strap trick every Muay Thai gym teaches on day one is simple: strap the lower band first, then the upper. Most beginners do it backwards, and that's why their guards rotate mid-round.
Sizing matters just as much. Measure from two inches below your knee to the top of your ankle. That span should fall within the guard's stated coverage range — not just approximate it. A guard that's too long will hit the top of your foot and torque outward when you kick.
For sparring, err slightly larger and wrap the gap with athletic tape. For competition, fit is everything — a guard that moves costs you a round.