indie, as I am handing her the tip, I will ask if its okay to give her as a reference ONE or TWO times. (More than two ref requests is a rude imposition on their time, unless you see them at least monthly.) They always say yes. However, I will give them as a reference within 1-3 months, not two years later. I NEVER ask a new girl on the first session if she will give me a reference for another girl. I think its tacky.
If you are an infrequent hobbyist, you have to be reasonable about reference requests. If you saw a girl one time, and never repeated, what incentive does she have to give you a reference? Likewise, if it has been six months or a year since you saw her, how can she even remember you? References are a courtesy from a provider, not a right, but you have to make it easy for them. If your hobby pattern is one-and-done, and she doesn't expect you will ever see her again, she has no incentive to give you a reference. If when you see her, you already have a name of who you want the reference for, so much the better. She will be ready for the request when she gets contacted by the girl you named.
Accordingly, I would recommend that you see a few girls that you have already seen first. Getting in should not be a problem unless its been a very long time since you last saw them. Then ask them during the session if they would mind providing a reference for you in the NEAR future. Make sure you tip well if they say yes. It implies that part of the tip is a good-faith deposit on spec that they will help you out with a reference. Then its up to their integrity whether you actually get the reference or not. If they are permanent SoCal girls, the likelihood is high that you will get the reference. With touring girls, they don't always come through.