It's not often you hurl your whole body down a mountain, so it's no surprise that when you go skiing you're using muscles you don't usually employ in everyday life. There are a few exercises you can do in the run up to a skiing trip that will mean your body is better prepared to move without strain or injury.
The Telegraph offers a good selection of step-by-step exercises to work on your biomechanics (your movement and structure) plus your proprioception - your body's positional strength - one of the main skills to develop in order to avoid injury as it helps your body cope in bad weather conditions (such as snow).
A couple of months before you go skiing it's a good idea to start doing regular squat exercises and wall sits in order to build up strength and endurance through your leg muscles. Pilates classes are also a good idea in order to strengthen your core muscles.
As with any advice on exercise, if you experience pain or discomfort during these exercises then stop what you're doing and get advice from your doctor.