1. Shock the System
First Step: Exercise the circulatory system and activate brown fat. To do it, go stand in snow barefoot. Or turn the shower cold and stand under the water for a minute. Whatever the source, the goal is to give your system a little shock, then suppress your natural response to shiver. (How? Take a deep breath, and relax. You're fine. You're not dying today.) Doing so sends a signal to your body to find another way to warm up; this activates brown fat and boosts metabolism.
2. Dial in breath.
To alter the way your body responds to external stress — say, standing in the snow barefoot — you need to train it to metabolize oxygen more efficiently. Power breathing is the way to do it. Begin by taking 30 fast breaths. Inhale for one second, and let the exhale flow out slowly and naturally. (You may start to feel dizzy or cold or experience tingling in your hands and feet: This is normal.) After the 30 breaths, finish with an exhale and time yourself to see how long it takes before you need to gasp for air. Hold on as long as you can, clenching the muscles in your chest, arms, and legs. When you can't stand it anymore, take in a half breath and hold it for 15 seconds. Exhale, then start over. Repeat this power-breathing process three times, increasing the length of the final hold each time.
3. Supercharge a workout.
Power breathing can temporarily increase the amount of oxygen your body has to use, allowing you to push harder during any short, intense exercise, like outdoor sprints. Here's how you can put it to use: After three rounds of power breathing and retention, do one final set, this time with 40 breaths. The extra 10 breaths should be at an even faster pace. After the final exhale, immediately do push-ups while holding your breath. Do as many as you can. The breathing prep will super-oxygenate your blood, making these push-ups feel easier than any you've ever done. The technique works for more than notching high reps of push-ups, too. Try it before any high-intensity workout.
4. Put it all together.
My daily routine mixes elements of all these practices. First thing after I wake up, I do three rounds of power breathing, followed by a breath hold. I time myself and try to add a minute to each breath hold until I hit three minutes. Then I do a fourth round of breathing followed by 50 empty-lung push-ups. I'll follow that with a headstand for 30 seconds, to allow blood to better circulate to my brain. Then I shower, starting with warm water and finishing with at least a minute of an icy spray. Afterward I feel refreshed and pumped full of endorphins. I also do some kind of outdoor cardio — no matter the temperature — three times a week. The plan never feels time-consuming, and it's also pretty much all I did to prepare to hike Kilimanjaro.