Ellen Wight (XXX Racing-AthletiCo) sends this one my way:

When you’re on a training ride and come to a stop — and your group ride always stops at red lights, right? — remember to shift to an easier gear. The gear you’ve been using to cruise at 23 mph is probably not the right gear to get you going from a stop. You’ll restart more quickly if you’re not overgeared, and the ride will flow better if the group doesn’t have to wait for a few guys to grind out of their 53x12. The same goes for races: Most packs will slow heading into corners, especially at the 4/5 level where people haven’t mastered pedaling through corners at speed. If you’re overgeared when you exit the corner, you’ll get gapped and eventually dropped. My rule of thumb is to always shift two gears lower heading into turns. This gives me the right gearing to accelerate out of the corner and close any gaps that may have formed.