You have two choices:
Sell on an exchange. (It is possible to send Tokens via http://www.myetherwallet.com/#the-dao or Mist)
Split the DAO. (Only possible via Mist)
If you want to split from The DAO, perhaps because you don't agree with a proposal or with the curators. It is a way to ensure the decentralization and autonomy of The DAO.
One should note that splitting is not entirely hard, but it has a number of steps and is time consuming. It takes (minimum) 7 days to Split the DAO and arrive at a place where your Ether and Reward Tokens are now in your complete control.
However, it takes a minimum of 48 days to "cash out" Ether into a standard account where you can send them, trade them, etc. You will need to take action, wait 7 days, take more action, wait 27 days, take more action, wait 14 days, and then you will finally have ETH in an account you control.
If you simply want to cash out, you may want to consider trading your DAO Tokens on an exchange rather than splitting.
The full instructions for splitting the DAO are found on the Wiki . It is long, but here is the overview of the steps / # of days.
You Need...
Mist / Ethereum Wallet: Downloaded, installed, and sync with the main net: Mist.
"The DAO" added as a Watch Contract: How to add and watch the DAO contract.
"The DAO" added as a Watch Token: How to add the DAO token to the MIST Wallet.
Your token address.
Some Ether at your token address to cover cost of gas.
Time.
##Then you...
Create a Proposal to Split the DAO
Find the ID of your Proposal to Split
Vote “YES” on your new proposal
Wait 1 week
Call the Split DAO Function – You are now officially split & have your ETH + reward tokens & are no longer part of the original DAO
Find out where your New DAO is and add the new Watch Contract & Watch Token
Wait the 27 Day Split Creation Period
Add your address to the whitelist (if you don’t want to send it to the new curator address)
Create a new Proposal to send all your ETH to an address
Vote Yes on the new proposal
Wait the 14 day debating period
Execute the proposal. Your ETH have now been sent to the address specified.
Again, read the full wiki for a step-by-step walkthru of how to split.