As i've always said, sandbagging in paid divs (sandbagging free divs is always loser behavior) only really becomes a major issue when u start giving any meaning to winning any div below adv. If u really don't want people to sandbag the first step to take is to stop giving prize pools and permanent recognition (badges etc) for winning lower divs.
By analogy, a sports player doing well in the minor leagues/collegiate level shouldn't feel accomplished because he's beating a bunch of mediocre players, he should be excited because he's got a good shot of making it to the league that matters (tho yes this analogy is obv relative to the size of the sport)