I think the way I would do it is have the cash to cover margin calls already in the account. I know the metrics are a bit different in that a stock (or ETF) can go up indefinitely, but can only go down to zero. It isn't so different in that every winning or losing deal in the markets involves a buy and a sell... just going at it the opposite direction.

What do you think of my idea of targetting the beta ETF's? To me it is like getting to be the casino for a change, where the odds are stacked more in your favour.