A lot of people fail to realize the abdominal muscles do a lot to stabilize the back. Weak abs are the primary issue for anyone with a persistent lower back pain. Core exercises are the best to strengthen the whole midsection, crunches are a waste and a thing of the past. If you get a chance to snatch up P90X (either paid or obtained via one of the wonderful torrent sites) and find the "Core Synergistics" workout, it is probably the best thing you can do for your back issues.
Riding a ski and doing a lot of jumping is rough on the back, and no amount of working out can correct a compressed or blown disc. The best thing at that point is to get the abs strengthened and take the stress off the erector spinae muscles that overcorrect when your abs are weak.
I will feel tightness in my back after a long day riding, and it will be sore the next day. It usually fades away after a couple days. It gets really bad with a full day of wake jumping. I am 42, so it comes with the territory. I should be doing some more stretching in my off days too.