Videos
When looking at videos, it's good to develop your 'eye' to see what you and other skiers do well and what needs work. Here are some things to look for...
Ankle and knee bend when kicking/pushing, hip position (are they over the feet), hand position (how wide, how far in front), how much crunch (is there a break/bend at the hips or are hips tucked under), how snappy/dynamic are the movements, what is the temp, what is the range of motion, etc...
Check out the RAHS Nordic YouTube channel for race videos.