Intervals - lots and lots of intervals.
And skills practice. On the bike, off the bike - repeat ad infinitum!
Seriously, though, intervals are what will increase your speed the most in a short amount of time. My favorite workout in the fall is to get together with a small group of friends, and do a "practice" race. We are lucky enough to have a location where we can go and do multiple loops of a "course" at full on race pace. Typically, we'll start with skills in the backyard - cornering, barriers - and then head out a local rail trail doing sprints on the way out (4 total that last about 2-3 minutes each with 30 seconds of recovery in between). After that, we head to the "course", spinning easy to recover. Then, it's 5 to 6 laps that includes a VERY long sand portion (great practice for cross) - each lap takes between 5 and 6 minutes.
These intervals REALLY help me a lot. I still ride other nights of the week, but this is likely my best workout.
SheFly
p.s.
Remember most of all, 'cross is all about having fun! And my typical mantra is that I can do ANYTHING for only 40 munutes