There's bitcoinlaundry.com, but I'm not sure it is effective. All the transactions are still in the block chain, so you could just compare timestamps and amounts. Am I wrong?
You need a service that will let you send multiple, random-looking amounts at different times, and then, some time later, send an amount to the destination. And then refunds any extra you sent (and you should send extra). And if you're really paranoid, it should allow you to give several refund addresses, and random amounts of coins get refunded to each.
Then you need to trust that service, which is the really big security hole. What better way to catch money launderers than to setup a money laundering service for them?
You're probably better off doing what has been suggested several times-- move coins between 'shared wallet' services like MyBitcoin or MtGox (and/or ClearCoin) or just between two machines of your own that are both running bitcoin.