Great advice on buying expired domains from Mike Industries. Basically, when domains expire they then go through a 40 day grace period, then a redemption period where it still can be renewed by the owner for $100. At some point the domain status is changed to "locked", which is a 5 day long phase at which point it will be available to purchase. However, there are three companies that are continually monitoring dropped domains so that they can be bought within seconds or minutes of being dropped....
Buying expired domains that are not competitive (low interest)
Godaddy backorder service - $18.95
Services that Help You Buy Expired Domains
Snapnames - Best choice for Network Solutions registered domains. Charge is $60 plus an auction is more than 1 bidder.
Enom - $30 fee to join "club drop"
Pool.com - Bid $60, and then moves into 2 phase auction
Check with the registrar to see if they have services similar to GoDaddy, such Tucows
Be sure to read the Mike Industries article.