Some things which have worked for me and may or may not work for you:
1.) Ensure you adequately rest.
2.) Do _rapid_ hikes up a long (1-2 miles) incline with a weighted pack. This had the benefits of increasing my leg and core muscle strength and increased my leg muscle speed. This is my single biggest bang for the buck, but learn to enjoy suffering. I do a local 2 mile, 30-45 degree incline with a 45lbs pack and haul ass with no resting.
3.) Cross-train on a bicycle or swimming. This improves performance, and helps speed muscle recovery and gives your legs something a little different to do. I tend to do these on my "rest" days of my running schedule.
4.) Do farklets. See:
http://en.wikipedia.org/wiki/Fartlek
4) Stay hydrated all of the time
5) Stretch your quads, hamstrings, calves, ankles, knees. Keep them limber.
6.) Get adequate rest. No, really.
Another technique I sometimes use on long runs when my legs are exhausted is shortening my stride, but running at a more rapid pace, so that you look like you're pedaling on a bike in too low (less resistance) of a gear. It takes some practice, but I'm able to run distances much faster while burning far fewer calories.
Hope some of this helps,
Jake