Everyone is different, so for all of the try this's and that's, some will work and some won't. One that has worked well for me is running with a heart rate monitor. There were days that I felt I was running "easy", but was really overworking myself. Most folks recommend avoiding back-to-back really hard days (over 75% or 80% max heart rate) and getting training in all three major zones in the week (i.e., interval for pure speed, tempo for endurance, and a long run for aerobic base). Knowing I was going way too hard at times has helped. I follow the 3-day per week "Less is More" plan from Runner's World and have seen significant improvement at all distances, though I started slow ... year over year improvement from 8:40ish to 7:15ish for my last 10-mile race.