Try with PreThink, if player's speed is 0, in next move (speed !=0) set trail...
edit: sth like that
PHP Code:
public PreThink(id) {
static Float:vel[3], bool:settrail[33];
pev(id, pev_velocity, vel);
if (vel[0] == 0.0 && vel[0] == 0.0 && vel[0] == 0.0) settrail[id] = true;
else if (settrail[id]) {
//message_begin(......
settrail[id] = false;
}
}