The "Pomodoro Technique" can be particularly useful. This technique was developed in the late 1980's as a means for time management. It typically means 25 minutes of work / followed by 5 minutes of break.

This will keep you relaxed, but also highly effective in those 25 minutes.

I love this method, and use it religiously. I even installed an extension called Pomello to help me stay on track. I strongly encourage you to test this technique and see how it impacts your productivity levels. For me, I find it allows me to concentrate deeply on a task, and then gives my brain a "rest". This I would say is one of the most important tools in my productivity toolkit.