I recommend www.pool.com for all your domain back ordering needs. It gives you a list of the domains that drop each day and the ability to back order them. The service is free but you do have to pay a higher price for your domain than a normal service provides. If there are multiple users who have backordered the same domain then it turns into an online auction.

I have tried and tested quite a few of these services and found www.pool.com to provide the best flexability and fast service.