CYCLISTS: A power meter, like CycleOps' Powertap Elite+ ($850; cycleops.com), is a more reliable gauge than, say, your heart rate or perceived exertion. It quantifies, in watts, exactly how much energy you are putting into your bike.
Garmin Forerunner 110 GPS watch, CycleOps Powertap Elite+ power meter
Garmin Forerunner 110 & CycleOps Powertap Elite+
RUNNERS: As long as you're on relatively flat terrain, pace is your best way to judge intensity, because it isn't subject to fluctuation from stress and fatigue the way heart rate is. Pick up a GPS watch, like Garmin's Forerunner 110 ($200; garmin.com), and use it to make sure you're hitting your target speed every run.