My coach has an article in Running Times on circuit drills. These drills as well as more hill work are the main additions to my training and I think they've made a big difference.
How big a difference? Well, I think my recent PRs exaggerate my improvement a little because I didn't race last fall except the marathon and I was really off my peak fitness that day. (I'd guess I was in more like 1:30-1:31 HM and 67-68 10M shape in October versus 1:29 and 66 this spring.) HOWEVER, I was running low 70s mpw last Fall versus low 60s. The way I'm running is qualitatively different now. I don't have any VO2max test to point to but I'd guess my big improvement has been in efficiency.
I thought miles and weight were the only real variables I had to play with, but now I've PRd while running less miles and weighing more (probably because I'm running less miles). I'm looking forward to seeing what I can do once I'm ready for increased mileage (and lower weight!).