Run, run, run, and run some more. That's the only way that you'll improve.
As for some small tips that'll help you in the long run (no pun intended), here's a couple:
a) When you run, try and roll your heels so that you aren't constantly pounding the ground and putting unnecessary pressure on your knees.
b) Take long breaths. The longer the period of time between breaths, the longer you'll be able to run since you won't be constantly gasping for air.
c) Keep your arms relaxed. I know it sounds silly, but if your arms are tense, you're wasting energy that could be used for that extra mile or so.
I was told these a few weeks ago, and I found them all to work very well. I hope these work for you, and anyone else who needs some help.