Never do static streching before any sport related activity, it does't lower your chances of getting injured. Warming and non-static stretching, however, does help preventing injuries! Stretching after excercise also does't reduce the risk of injury; technically, if your muscles and antagonist muscles (for exemple Biceps and triceps or quadriceps and iscios) are of equivalent strengh, you should not get injuried. This, on the other hand, will prevent you from increasing your flexibility (since you're not stretching: duh!) so if you think you need flexibility for any other activity, strech before (stretching before reduces your muscles strengh by 10-20%). Also, drinking enough water prevents dehyddratation, which increases the risks of injuries and muscle cramps (and everyone knows that sucks).
As soon as you get injuried (muscle/ligaments/etc. wise, this doesn't apply to broken bones, put some ice (or even better, if you can, submerge the injuried part in ice cold water) on it for 15 minutes. This will reduce inflammation and will drastically reduce your recovery time. Never put hot packs on injuried muscles within the first 48 hrs of the injury, this will increase inflammation and thus make recovery time much much much longer!
Eating proteins right after you're done with your ski day will decrease immensily your muscles recovery time (you'll get less sore). Eating within 30 mins after the exercice is the optimal time, otherwise, the sooner the better. You'll also need some carbs for your body to effectively use the proteins (gellybeans or any fruit/veggie will do fine, they are easily transformable carbs).
Never attempt intense stuff when you're tired, this is the best thing to do to get injuried. This is because your muscles are tired and don't protect your ligaments as good (can someone say broken ACL's?). When your muscles are fresh, they will protect your ligaments more effectively.
Alcohol dehydrates the body, wich increases the chances of injuries and cramps. This only applies if you do some hardcore stuff, plus drink some water with your beer/vodka/rhum and you can get drunk and not get injuried (pro tip for ya drunkies). Being drunk also makes your muscles more relaxed, this decreases the intensity of injuries (true story, that is why drunk drivers usually get less hurt that the people they hit)
If any of you are skeptical, I can link to the PubMed atricles.
TL: DR? Well fuck you and get injuried, suck a fat one and don't progress.