I wish I knew!

1. Just about every PSE has a friend who escort UTR.  WHENEVER you see a pornstar escort, ASK her about her UTR friends. If she had a good time with you, she'll be willing to share her info, and you have a built in referral.

2. Bite the bullet and use one of the agencies that specialize in UTR girls.  It may cost you more than if you saw her direct, but it gets your foot in the door.  Once you've seen her, try to get her direct contact number for the next time and again, ASK her about her UTR friends.

3. Try Myspace... send PMs to the hot models, tell them whom you've seen in the past, and ASK.  You'll get a lot of rejection, but you may get one or two yeses.

The whole key is to get your first one or two UTRs under your belt... then if you remember to ASK, it gets easier and easier.