Take breaks

 If you schedule your day in advance, chart out some time to take breaks from your work, and plan your days off far in advance. Or set an alarm to remind you that it's time to take a break. There's no established interval for this; some studies suggest that the seemingly random pattern of 52 minutes working followed by 17 minutes breaking is especially effective, but each individual will have a different preference. The key here isn't to follow a specific type of plan but to have a plan in general, so you're more likely to follow it.

Break even if you feel like you don't need it

If you're on a hot streak and you're blazing through work, you'll have the urge to keep going no matter what. On the other hand, if you know you still have a lot of work to do and you're tired, you might motivate yourself to keep going despite an increasingly sloppy performance. The line here is blurry, so as a general rule, you're better off taking a break even if you don't feel like you need it. If nothing else, this can give your mind a chance to rest and recover before you ever hit that barrier of fatigue.

Disconnect entirely

Some people "take breaks" by sitting at their desks, leaning back in their chairs, and catching up on emails or text messages. This is bad form, as it's not a real break. Your mind is still occupied, you haven't moved, and your eyes are still being worn down by the screens in front of you. If you want to earn the benefits of a real break, you have to disconnect from the digital devices and communication streams you've been connected to throughout the workday.