Run, run, and run some more...
Cross training could include biking, swimming, etc. However, the best way to get better at running is to run.
I've tried all kinds of training regimes over the last 3 years, but the thing that worked best for me was simply mixing up my workout during the week with long endurance runs and short strength/speed workouts. (e.g. once a week I run a 10-12 miler and a 7-8 miler, and the other 5 days I fill up with sprint work at the track or fast 3-4 mile runs).
Cross training is good stuff for an advanced athlete, but I'd advise you not to neglect your running-- that's what's going to make you faster.
Just my 2 cents.