I'm a strength coach, and I thought i'd weigh in on the subject.
Cycling is a great way to keep in shape for skiing, however, it has it's limitations. Pros: lighter impact for off-season recovery, aerobic, leg strength. Cons: not very ski specific, V02 peak is only possible (as opposed to V02 max or the point at which your body uses as much oxygen as possible, cycling can only reach peak because it does not encorporate the upper body musculature). As with any exercise, intensity/intervals are very important. Cycling up and down hills with easy stretches is more ski specific and trains the body to use similiar energy substrate patterns. When you ski, realistically it is only for a few minutes at a time before you stop to rest or wait for your buddies. Training should be this way, regardless of the mode of exercise. Boxing is also a great tool, because it is intense enough to build up lactic acid, but with simulated rounds, it can mimick similar stop/go patterns in skiing. The bottom line is that training has to be fun, so if you love cycling, obviously it is a great thing to do if the other option is sitting in the office.
I would recomend strength training and other exercise types along with cycling. Exercise programs is a puzzle, and you need more than one piece. I read some earlier posts for and against core training. Some of the arguments seem to have come about due to a lack of consistency in the term 'core'. Bosu ball, stability ball, and other exercises of this nature, in my opinion, is 'core balance'. This is important, however, this must not be confused with strength. These exercises are extremely challenging but can be modified for any level. Strength and endurance are the main goals, however, without proper balance, it means nothing. You can bench press 500lbs, but can you duplicate that strength without a bench supporting you? Bosu balls and other balance training can be thought of as the base. The broader the base, the more room to add strength that can be functional. If you have faulty movement patterns, performing strength exercises (especially multi-joint lifts) can be detrimental. Think of it this way: by putting a corvette engine in a 2 door Kia, it would destroy the structure of the car because it is not equipped to handle it. By building your base of balance and strength in the deep muscles of the trunk and hips, you are better equipped to handle the bigger lifts and you can apply that strength better.
Now, for strength training:
I would shy away from the typical bodybuilding format, 3-4 sets, 12-10-8 etc. While this may be good for hypertrophy (muscle mass inc.), it is not very functional to skiing. I would do more of a multi-joint approach, alternating from high intensity low rep to low intensity high rep. Skiing is unique in the fact that it requires a high amount of strength but also endurance. In a typical training format, endurance is built first, with lower reps/higher intensity coming after, and ski specific skills coming toward the start of the season. Of course, this is a blend that requires personalized attention. In-season, I would say light cross training and stretching would be most appropriate.
As for stretching, I have read some posts against static stretching. I disagree with this statement. If your flexibility is not up to par, static stretching is a great tool. Holding a stretch 15-30 sec. and increasing range of motion is a must in terms of injury prevention and reaching your full muscle potential. This comes back to movement patterns. If you have a lack of balance or tight muscles, it puts your body in a biomechanically poor position and you are destined to eventually fail or at least not reach your potential. Now, of course, if you can incorporate active stretching into your routine, great. Yoga is a great way to do this. It really comes down to your schedule and level of flexibility, but it is not to be ignored!
Hopefully this makes some sense and it can help some of you out!
Feel free to message me privately! ideafit.com/profile/marc-halpern
Have a healthy rest of the off-season!