Larry Feldman wrote:
Tie some knots in the end of the drawstring, and it will help prevent the drawsting from 'disappearing' into the pants.
Another tactic is to sew through the drawstring at the front of the pants so that the drawstring won't move.
I can't say I've worn a drawstring-based pair of dogi pants in quite some time. I very much prefer the clasp-based pair of dogi pants that I get from Bu Jin Design. I've had too many drawstring-based pants stay knotted at the worst possible times (eg need to go to the bathroom!) due to the drawstrings being drenched with sweat. They're quite a bit more expensive, but they last for years (especially under a hakama).