My wife has the following mileage balances:

American - 20759 (expires 8/05)
Delta - 16427 (expires 12/03)
Continental - 6479 (expires?)
Northwest - 19222 (expires?)
United - 14307 (expires??? - last activity: December 2000)

I couldn't seem to find any expiration info for CO and NW, do these expire? Also, I am assuming UA will expire at the end of the year, but I couldn't find any details.

What is a good use of these miles? (Or what is an easy way to extend the expiration so that they can be useful.)

(She is also not likely to fly a paid flight on any airline in the near future.)