Spring, summer and autumn give you a good seven or eight months to get ready for next winter. There are many approaches, several of which I have written about before. The ideal combination would be a balance of strength and power training, aerobic training, balance and flexibility training. It is best to begin with building an aerobic base at the start of the off season, then gradually introducing weights work to build up strength particularly in the legs and core, before adding speed/power training at the end of the winter. Balance training can be added into the mix at any point.
I have mentioned slackline before - it is great for balance and core strength and you can make up exercises or tricks to train particular muscles. I like to try one legged squats on it for example.
Another unusual training aid is the unicycle - great for balance and working legs and core.
Gym work is also important for that strength training. I wrote a series of posts on ski fitness a while back which are still relevant.
Being creative and imaginative can really help. There are many exercises using different equipment, or none, such as swiss ball exercises, plyometrics, balance boards, jumping or hopping over a ski pole or line. Search YouTube for some top skiers' training routines if you need inspiration.
Finally I will leave you with Jonny Mosely dryland training bumps. The video is a little old now but it is still awesome to watch, and demonstrates how a good training session can use the same movements as skiing.