Dude, that sucks that she bailed. Did you have her sign a contract before starting the vid? With a signed contract you could at least prove to sexyjobs or others that an agreement existed between the two of you. Of course she could still deny being paid, but the fact that you have some footage of her would support the argument that you either paid her or she was willing to be paid after the video was over, since why would she begin performance of her duties under the agreement if she thought she wasn't going to be paid?
In a legal sense, TF's contention that he was ripped off isn't hearsay. He was there and experienced it first hand. Just for purposes of illustration, if he were to sue the model, generally he would be allowed to testify even about what the model said (in a non-legal sense we would call this hearsay), and her lawyer couldn't successfully object on hearsay grounds.
This isn't legal advice, but what I would do when producing your next video is have each model sign a contract on camera, then say something like, "so you just agreed to be in a tickling video, right?" and she would say yes. You could also film her being paid, and you could have her acknowledge that the money was being given in consideration for her services as a tickle model as specified in the agreement she just signed.
As to the issue of whether the model gets paid first, I've heard of many photographers paying the models immediately after the shoot was done. Having a contract signed by both the model and the producer that specifies exactly how much the model will be paid and that she is to be paid immediately after the shoot would make the model feel a lot more comfortable in terms of trusting that she actually will be paid. With that said, half up front and half immediately afterward seems pretty fair and convenient for both parties.