Interval training!! Found some GREAT podcasts (GetFitPod) and my dad and I did one this morning. We only did about 3/4 of it because I really needed a pit stop--looking forward to doing the whole thing next time.
The interval training I did alternates runs (2 1/2 to 3 minutes) with walking (30-45 seconds), and within the runs are occasional short (15-45 second) speed bursts. I'll try to do interval training every week or so, and it should really help my 5k speed. Skip Orem's GetFitPod offers a bunch of different free interval workouts (plus informational podcasts with other useful fitness info.)
The idea behind interval training is that you really push yourself aerobically, and you increase your heart rate, during the speed bursts (and somewhat during all of the running portions, which should be a little faster than your normal running pace.) Your heart and lungs get stronger, which allows you to eventually go faster on long runs.
This was fun and energizing, and I can't wait to do it again! For those who do the Couch-to-5K program, most of that is interval training! When you complete it and are doing long runs, I'd suggest adding these podcasts (or another interval training plan) into your routine.