I just went through this myself so figured I'd share with everyone else.

Poloniex is crediting ETHC into accounts who deposit ETH to their Ethereum deposit address. This is of course if the account you're transferring from has ETHC balance attributed to it. So here is a step by step guide of what I did (adding 7% to my ETH stash in the process).

1. Check the balance of your account on the ETHC chain at http://gastracker.io/addr/****** Just fill in your wallet address instead of the *****, but make sure all letters are lower-case, as the site throws an error if you copy the address in and the letters are upper case.

2. Login/Signup for Polo and deposit your ETH on the MAIN chain to Poloniex's Ethereum deposit address.

3. Wait a few minutes and your ETH deposit will show up in the deposit history. A couple of minutes later you will see your ETHC deposit show up as well, assuming you had a balance in the account you sent from.

4. Sell ETHC for ETH directly (currently around 7% value). Withdrawal back to your account on the main chain.

Simple enough.