Lift weights. You don't need a gym for this. Buy some cheap dumbbells from Walmart. If you're a poorfag, fill up old milk jugs or tote bags or a backpack and lift that. Add some crunches and squats to the mix.
I used to laugh at this standard /pol/ advice until I actually tried it myself. Within minutes, my mind is calm. I feel physically and mentally better for the rest of the day. The hardest part is getting yourself to actually exercise, but once you get started, it's easy to keep going.