In order to send Ether, you need to pay a transaction fee of 21k gas (more if you are sending to or from a contract-based wallet).
Mist rounds some Ether values, so it is very difficult to send exactly the amount in your wallet. To send roughly the amount you want, just lower the transaction value by 0.0001 ETH or so, or reduce the transaction fee by sliding the "select fee" slider left.
If you really want precision, you can use the command line client that is packaged with Mist. Open up your command line, and cd
into the ./resources/node/geth/
directory. Then run
./geth --exec "eth.sendTransaction({from:'0xADDRESS',to:'0xRECIPIENT',value:eth.getBalance('0xADDRESS')-21000*50000000000-1,gas:21000,gasPrice:50000000000})" --unlock '0xADDRESS' console
Replace both instances of ADDRESS and RECIPIENT with your address and the recipient's address respectively. Leave the quotation marks.