KIT33R said..
Try curling your toes up to lock yourself into the footstraps. I ride in really loose straps on a dead flat pad and this works for me.
Interesting, I curl my toes
down to push the top of my foot higher into the straps.
Losing the board used to be an issue when I first started boosting, but now I'm hardly ever losing it (without bailing the board on purpose) with no extra tightening. Pretty sure it's just confidence and practice.