If you really want a domain and you're sure other people want it too, and will be placing backorders themselves, make sure you place a backorder at all the top 3 snatching services; Enom, Pool and Snapnames.
Else just go with Pool.com. They have the highest success rate.
Also, If it the domain is registred with Network Solutions, make sure you go with Snapnames because they have a partnership which gives them priority for repossessing expiring domains.