- Half-hour on, Half-hour off
This works like you wouldn't believe. For half an hour you work with no distractions, then you take the next half hour to do something else: read e-mail, blogs, go for a walk, get lunch, take a smoke break, play video games, watch TV, anything. Keep flipping between these modes all through the day, and you'll get 12-hours of work done in the space of 8, even though you were only ostensibly working for 4. You'll be more relaxed, your subconscious will continue working on problems during the half-hour off, you'll be able to concentrate better, you'll eliminate "internal distractions" because you know it's only a few more minutes before you'll get to them, and you'll keep your eyesight from degrading by refocusing at regular intervals through the day.
- Naps, not energy drinks
Energy drinks put you on a chemical roller-coaster, but a nap eliminates more than just the feeling of being drowsy. When you feel tired, use your next "half-hour off" to snooze.
- Stress is the ultimate carcinogen
Whatever you do to relax, it can't be worse than the alternative. Avoid stressful environments, even if it means finding another job.
- Park on a downhill slope
Try to end each day with something easy still left to do, like finishing off some documentation, banging out some routine getter/setter methods, or refactoring a bit of messy code. When you come back to work the next morning you'll have a low-resistance task that eases you back into the flow and sets up the momentum you need for heavier tasks later.
Regular exercise has always been superior to stimulants because it increases blood-flow to the brain, stimulates neurogenesis, and keeps your heart-rate up through the day, so do it in the morning or on your lunch-break. If your fat-ass is too heavy to run because it hurts your knees, try using the Elliptical machines at the gym.