To make it tighter try this:
with your pants on lay your back on the floor, mat etc.
Take the two sides of string into your hands
Lift your hips off the floor
Pull the strings back and forth, like you normally would, to get them tighter.
With your hips still in the air and still pulling the strings tie them as tight as you can.
Basically, by doing it this way you remove the friction that prevents you from getting the pants tight enough. They get much tighter and shouldnt come off this way.