I shorted a basket of leveraged ETFs for a while.
Short them in pairs so TNA & TZA, NUGT & DUST, etc. so you're always long & short in the same market.
In the long-run you'll win. The problem is you need to keep a good amount of spare margin available because occasionally these leveraged ETFs go crazy to the upside. Short @ 10 and suddenly it's 100.

Initial investment was 10K and now it's 100K. Yikes!
I was profitable with the strategy but with a strong move it can take considerable time to get back to break-even and with the spare margin required I decided it wasn't worth it.