I haven't read the bogleheads thread yet (TL:DR), but if someone put a gun to my head this is what I would do:
1) Buy index futures, it offers an incredible amount of leverage with no interest rate cost. Only costs are trading commissions.
2) Interactive brokers charges roughly $2 a trade commission per futures contract
https://www.interactivebrokers.com/en/index.php?f=commission&p=futures13) Right now the S&P 500 is at 2066. The e-mini S&P 500 contract size is $50 x S&P 500 price
http://www.cmegroup.com/trading/equity-index/us-index/e-mini-sandp500_contract_specifications.html That means the contract is worth $103,300
4) Each point of the S&P 500 is worth $50. That means if you buy 1 contract and the price of the S&P goes down by 5 points, you just lost $250.
5) The margin requirement (currently) of the e-mini S&P 500 contract at Interactive Brokers is $4,600. If you have a $5,000 account and buy 1 e-mini S&P contract, if your account gets to or below $4,600 they will automatically sell you out of your position. With only $400 difference that's only 8 S&P points, not much room for error!
6) But if your account was $50,000, and you bought 1 contract controlling $103,300 worth of the index, that's 2X leverage and no interest to pay. For you to get margin called, you'd have to lose $45,400. That's 908 points for the S&P 500 to lose before you're stopped out (and you've basically lost all your money....).
7) Because that's about a 50% loss for the S&P, if we had another financial crisis or whatever you could lose all your money if today is the top. THAT'S WHY YOU ONLY DO THIS IF THE INDEX IS DOWN HARD. I'd wait for at least a 20% pullback before attempting this. That would increase the odds that the S&P won't fall enough to give you a margin call.
The problem would be that the exchanges can increase the margin whenever they want. If they increased the margin to $10k, then that's less points the S&P has to go down before giving you a margin call. Another issue is that the contracts expire every so often, and you have to sell the contract and buy another one out in the future. That increases your commissions and also creates a bit of slippage. The spread/tick is .25/point so the bid price is 2066.00 but the ask is 2066.25. If you sell, you have to sell at 2066, if you buy, you have to buy at 2066.25. So you're gonna give up at least .25 a point every time you 'roll' a contract as it's called. This may not sound like much but it adds up over time. If you just leverage up an ETF, you could hold onto it indefinitely and never have to sell it, reducing the slippage to nothing.