Have someone videotape the lesson. You will likely get thrown a lot of information, so it's nice to be able to re-visit the lesson at later dates and really absorb everything you learned... again. I do suggest treating the clinic a little like a show. Dress well (clean breeches, nice shirt tucked in with a belt, clean boots, hairnet, gloves, and helmet), have your tack and horse clean. Some clinicians are more sticky on appearance than others; it's best to play it safe. Don't wear anything baggy; the clinician will want to be able to see your pelvic rotation and back. Warm up for 10-15 minutes before the lesson, get your horse's muscles supple and warm. Most of all, enjoy, and learn lots!