It's very difficult to rely on our ability to catch a domain name as it has dropped and register it before someone else does. Furthermore, most of the registrars don't even allow the better domains to be dropped, instead, placing them in online auctions. This is where backordering comes into play. With backordering services, you are able to pre-purchase a domain name that is about to expire, as long as the current owner fails to renew it themselves. However, there is no guarantee you will be given the domain name, even if it expires due to other back ordering services offering the same opportunity to other people who are looking to snag a great domain name that's about to drop. Still, it definitely increases your chances at being able to scoop up some of the better domain names with existing traffic and page rank. Here are a few of the more popular back ordering services:
www.NameJet.com
www.SnapNames.com
www.Pool.com