As katherine mentioned you can either buy the domain from the owner or snag the domain before it is deleted.
It's important to note that not all domains backordered at Namejet, Snapnames, etc. will retain their age.
When you find a domain you want make note of the registrar.
Some registrars have relationships with specific auction houses. You can use a google search to see if expired domains from one registrar go directly to a specific auction house
(e.g. NetworkSolutions domains go to Namejet).
Some registrars have backordering also have their own backordering services (e.g. Godaddy),
but unless the domain is registered with Godaddy (use Godaddy auctions to snag the name) it's better to use Snapnames, Namejet.
At namejet and snapnames you have to bid on Pre-Release domains which are auctioned prior to the domain entering the pending delete phase.
If you are successful and win you get the domain and it retains it's age.
However, any domain that is 'pending delete' will not retain it's age (even if you use Namejet, Snapnames, etc).
Hope that helps.
Skinny