Quote from atticus:
I like very cheap ~atm long flies:
SPX 1330
Long 1280/1310/1340 from 5.75 mid, APR monthly expiration. You're actually long gamma atm and become short as you approach neutrality. It's all the curvature you want from the outset. The gamma sensitivity is very close (in slope) to the PNL at flat vol. Of course, vols will rise a bit as we drop.
I also like to team it up with a long calendar at the neutral strike. You're not shorting any appreciable gamma in the calendar, but it will increase net-gammas as it trades your way. It's a means of minimizing vegas as you trade to your neutral strike. It's somewhat self-regulating. You will begin to get uncomfortable with your gamma position right about the time it's optimal to get out. If it touches the center strike you're looking at a double.
Sure, you lose on the upside, unlike the backspread, but I'd rather be wrong and see it in my position than lose opportunity in a backspread creeping to the neutral strike and lose on time/synthetic vol.