One guideline is to try to lure the hostage taker into a false sense of security.  You also want to try to open lines of communication.  If you get the hostage taker talking, you can often get him or her to let hostages go or surrender.

Whatever you do, do not agitate them. Offer incentives to release the hostage, but not at first. First you would have to establish a relationship with the hostage taker, hopefully even trust. Promise them something you know you can deliver on, and make it happen. As the previous response said, you would be stalling for time, and your first priority would be keeping things as cool as possible.