How do you build a pool of applicants?

I sometimes run into the problem of needing to replace someoe on short notice - not often, probably less than once a year. How have other people kept possible applicants "in the pipeline" for a long period of time?

Wendy,

One model I have seen is based on somethingthe University of Phoenix developed. Schools keep running recruiting and training sessions even when there are no open positions. This helps them identify those who they may need on short notice.

Jeffrey Schillinger