Mike Bond from the USSA Center of Excellence presents a great progression of strength and conditioning exercises well suited for traveling on the road or doing at home with little or no equipment. No need for a gym pass!
There are a couple different tests out there to test fitness. The first is the Medals Test. A second is a variation used by Cross Country Canada. This test is something you can do as well. You just need a friend and an access to a gym or par course.
H.C. Holmberg's research founnd that doing 20 second doublepole intervals had three positive effects: Doublepole economy increased, 30 second doublepole performance increased, and 6 minute doublepole performance increased.
The area where most racers can make grand strength gains is in specific strength. Specific strength is carried out on rollerskis and on skis. It is doing long distance efforts as well as shorter (interval like) repeats using the upper-body only.
This article on strength and power for cross country was written by Zach Weatherford, a USOC strength coach who works closely with US Cross Country Ski Team athletes and coaches. You will find an example workout based on his essay.