How do I prevent overheating in heavyweight base layers?

Three strategies: First, choose a half-zip or quarter-zip top so you can dump heat quickly during exertion. Second, look for body-mapped construction with ventilation zones under the arms and along the back. Third, use the 'start cold' principle—when you step outside, you should feel slightly chilly in your heavyweight layer. You'll warm up within 5–10 minutes of activity. If you're comfortable standing in the parking lot, you'll be overheating on the first run.