There really are some great answers here, and the common denominator I read from the ones who are selling the private lesson are:
1. deliver of the product and
2. create a relationship through great service and commitment.
Then the lesson sells itself.
However, it is really baffling to me that so many instructors refuse to mention anything about their availability for the next lesson--either private or class. Simple phrases like, "I've really enjoyed skiing with you today. The next time we ski together, we can really build on what we did. If you feel like you need some more help, I'm available whenever... etc. If note, well just have a great time, and come back and see us soon.
When you're an instructor, the area is your home and gracious hospitality includes an offer to return.
As for price, look at it this way.
Just because I can't afford to pay that much, doesn't mean I'm not worth that much.
Secondly, if you don't think you're worth that much, you better start training or get a different job because you're ripping off the guest and the company.
Third, if I could afford it, I'd easily pay much more than the daily all day price of a lesson for a day of skiing with, say Bode Miller, or Janica Kostelic. If I've done my training and paid my dues as an instructor, then I have the ability to offer my clients the same relative benefit that Bode would offer me.
Victor Gerdin used to tell pros to take a good look inside at the end of the day and decide whether they were worth the money. If so, great. If not, fix that.
Lastly, don't under-rate yourself or the product you offer. Humility is important, but the joy and benefit that great pros offer is priceless. And on top of that, the guest gets lift line privileges. Wow. What a deal!