It's easier to get packages this way, rather than flights only; sites to check are site59.com and lastminutetravel.com. Calling a local
travel agent will sometimes turn up last-minute deals, but this is more common w/ cruises.
Hotwire and Priceline can also work, but it's pretty risky, you can end up going down late at night and coming home first thing in the morning, with layovers.
For flights, if you subscribe to airline emails, you can be sent notice of last-minute weekend sales. I subscribe to these on AA, USAir and United, but the catch is that you almost always have to purchase the tickets by midnight Thursday, and you usually have to return home on Monday or Tuesday, not Sunday. Sales can be great, though.
BTW, if you do get something, get to the airport earlier than normal; buying last-minute marks your boarding pass for a higher level of security.