1) Talk to younger children about taking a lesson. Ski and Snowboard lessons are designed to be a fun experience but lessons, especially the first one, can be scary for a young child. If they know what to expect it will less intimidating. Let them know how much fun it will be.
2) Be aware of your (or your child's) limitations and set your expectations accordingly. Many things can impact success; skiing and snowboarding can be physically demanding, especially as you begin and fear often plays a role in most students' ability to learn. By acknowledging limitations and setting realistic goals you will be better prepared to have an enjoyable experience.
3) Give yourself plenty of time. Rushing to get through the ticket line and the rental shop adds way more stress than you want before a lesson (or at any time for that matter). By giving yourself some extra time the whole process won't seem so hectic.
4) Wear the right clothes. The surest way to ruin a lesson is to be cold and/or uncomfortable. With that said there are a few tricks that seasoned vets use to stay warm and comfortable.
5) Make sure that you equipment fits well, is the proper size and is properly adjusted. If you are renting your equipment the rental shop staff will help, but as a rule ski boots should be snug but not painful. Having boots that are too loose will make it more difficult for anyone to learn and boots that are too tight hurt - neither of which will make for a good experience. Skis and Snowboards should be properly sized as well. While borrowing hand-me-down equipment might seem practical - if the skis aren't sized properly (between chest and chin for beginners) it will be very difficult to be successful. The price of renting properly fitting equipment is well worth it when it comes to having an enjoyable experience.
6) Practice before hand. For kids, there are many things they can do to be better prepared. If there is snow on the ground at home make sure they play outside some in the days before the trip to the ski area - after all skiing and snowboarding is just a way to play outside. If this is their first trip have them stand on a piece of paper and practice turning their toes inward (making a pizza slice), pointing their toes outward (making a V) and pointing the toes straight ahead (making french fries). Games like this help them to be used to turning their feet. For Adults think about what you want to get out of the lesson and you reasons for taking one to begin with. Having a little insight into your motivation will help your instructor teach you what you need to know.