But I think one of the overlooked things about the taper is the importance of intensity. When we think about relaxing and resting up for a race, it's often thought that means less mileage AND slower, easier runs. But that isn't the case. In fact, studies have shown that keeping intensity high (while decreasing volume, i.e. decreasing mileage) yields the most effective taper. In a meta-analysis (basically a summary and evaluation of the existing research) published a few years ago, Bosquet and colleagues looked at the variables that contribute to an effective taper. They looked at swimmers, cyclists, and runners and considered changes to training volume, intensity, and frequency and differences in the duration of the taper. Overall they found a two-week taper with a 41-60% decrease in training volume to be the most effective. Additionally, they found decreases to frequency (number of workouts) had no effect and decreases to intensity were either not effective or defective. In summary, it seems the best taper consists of running the same number of days at the same paces (some easy days, some faster days, etc.) as training, but less total miles per day and per week. (Also, it seems the optimal duration in general is two weeks, although other studies looking specifically at marathons agree three weeks is better for that event.)
I think the most important lesson here is the value of intensity; all the studies and reviews I looked at agreed on its importance and I think it's often ignored. We need to be sure we are well rested but still fit on the starting line, and reducing total mileage while still maintaining intensity (keeping up with our strides, track workouts, short marathon pace runs) will do just that. Additionally, keeping intensity high will help the mental aspect. Often the taper can lead to fears of losing fitness from running less. Having some speedy (but short!) workouts can help to assure you your hard training is not going to waste.