You didn't say what type of skiing you do, so I will reply to this as if it could be any. For myself, I do all types of skiing (alpine, backcountry, XC, nordic skate). I find that a combination of cycling (both on and off road) and running during the off season is a great way to stay in shape for the ski season. It isn't a perfect science, however. For example, I still find that my quads need a bit of extra work when I start the season again, but a few weekends of skiing usually bring those up where they need to be. If time is a factor, you can easily take a few spin classes and run in your neighborhood.

If you backcountry ski, another idea is to hike with a heavier pack during the off season. If you are a nordic skater or XC skier, you can also get roller skis and use those around your neighborhood when you have time. It's a lot of fun and can be a good conversation starter with the neighbors 😉

In general, I find that if I keep my cardiovascular fitness up, it sets me up with the right foundation when the ski season starts. Trail running, mountain biking, road cycling, etc. are all great ideas (and a bit of yoga too to help stretch what will certainly get tight as you increase your activity level). 

Have fun!