Sine I have gotten 2 separate PM's regarding the proper tipping custom for Ski/Ride Instructors I thought I'd post publicly.
Yes most people tip the instructor. The amount varies dependent on private versus group, kids, little kids, adults. Yes it can get confusing.
For most groups $10.00 per person is OK, more if you've left a little one kicking and crying and generally being a pain. If they did not call you to come pick up the little darling then you know the instructor did a great job! Consider tipping a bit more. If you are lucky enough to get a private lesson when signed up for a group consider tipping more too.
In a private lessons tips are more $50.00+
Some people feel awkward passing out bills, if you do then bring along an envelope to put the cash in.
In the end it is up to you. If you had fun and improved to me that is worth a little extra $$$Edited: 2:39 pm, December 16, 2010