How to Conserve Energy During the Downhills
Weather and the pre-race routine are important, but they are dwarfed by considerations for the key element of Boston that you can control: the hills. While Heartbreak Hill at mile 21 is the most notorious of the bunch, it's actually one of the less significant factors in the race. You have plenty of opportunities to ruin your race before Heartbreak Hill, so you must take care from your first step out of the chute to manage your energy use and the stresses on your muscles.
As you build your race strategy, the first decision you need to make is how to handle the 4 miles of downhill at the start, through Hopkinton and Ashland, with a net descent of around 360 feet (including 130 feet in the first half-mile). The obvious temptation is to try and bank time here, since that type of descent can easily help you gain about 15 seconds per mile or a full minute early in the race.
This would be a bad decision, as taking these hills too quickly—as with taking any start of a marathon too quickly—will create issues for you later, and every 10 seconds you gain here could easily cost you three times that later. Taking the hills too fast by allowing yourself to overstride will trash your quads, and this will really have an impact when you do hit the uphills (and then again when you have nothing left for the last 5 miles).
The right strategy for the first four miles is to run controlled, using good downhill running techniques—shorten your stride, don't lean back, avoid "braking" effects. Stick to your marathon race pace, or even slightly slower. You are doing things right if everyone else is passing you early, particularly if the runners from the corral behind you start catching up. A high percentage of runners go out too quickly at Boston and ruin their race. Don't be one of them. Save your energy for the hills that come later, and a fast finish.