A warning to anyone who isnt a very experienced trader: don't margin trade. You have better chances of losing your BTS than anything else.
Note #2: If you are able to lend BTS that you have on polo for margin interest, then this is a good way to safely grow your BTS. ("Safe" as long as you trust poloniex that is). I wouldnt recommend buying with borrowed money or shorting however, unless you are a pro.