Mental energy is precious, so learn to say no in order to protect this resource.

If you spend your days interacting with a computer, there's a good chance you've encountered those times when you just feel stuck, staring at your computer screen. The more you try to get unstuck, the less you feel able to make any decisions. This is due to the fact that, when your mind is exhausted, no amount of time or effort spent on contemplation can help.

The key to avoiding this kind of exhaustion is to have mental energy, as this is what allows you to be enthusiastic and enjoy what's going on in your professional and personal lives.

But while it may be important, mental energy is also scarce and easily depleted.According to a 2007 study in the Harvard Business Review, the average person enjoys only two hours of peak mental focus every day, along with an additional five hours of relatively high mental focus. At all other times, there's a good chance your mental focus will be relatively poor.

So, how do we make sure our mental energy gets replenished each day? A 2012 study by medical researcher Taeko Sasai suggests that sufficient sleep is what's needed for this to happen. But even then, with high mental energy and especially peak mental energy being such limited resources, it's clear we need to treat each minute with care.

And that's where Selective Focus comes in. Selective Focus is about being careful and choosy about how you spend your energy, and the first rule is learning how to say no to certain things that are competing for your attention. After all, you can't give your time and energy to everyone, no matter how politely they ask.

Saying no doesn't always come easily or naturally. Many of us were programmed as children to say yes to whatever our parents or our teachers asked of us. When you said yes growing up, you were probably rewarded with attention, praise or even a big, welcoming hug.

But now that you're an adult, there are rewards to saying no. Through analyzing over 80 studies that looked into the benefits of saying no, psychologist Martin Hagger found conclusive evidence that it not only helps people avoid wasteful and unproductive activity, it also helps them achieve their goals more efficiently.