Let's get this out of the way: Mental toughness is not a quality you either do or don't have.

Sure, some people may have greater self-discipline than you possess. Some people may be better at resisting temptation than you are. But that's probably not because they were born with some certain special something inside of them--instead, they've found ways to develop mental toughness and use it when it really matters.

They're mentally strong because they've learned how to be--and you can too.

Here's how:

1. Always assume you are in complete control.

There's a quote often credited to Ignatius: "Pray as if God will take care of all; act as if all is up to you." (It's a very cool quote.)

The same premise applies to luck. Many people feel luck has a lot to do with success or failure. If they succeed, luck favored them, and if they fail, luck was against them.

Most successful people do feel good luck played some role in their success. But they don't wait for good luck, or worry about bad luck. They act as if success or failure is totally within their control.

If they succeed, they caused it. If they fail, they caused it.

By not wasting mental energy worrying about what might happen to you, you can put all your effort into making things happen. (And then, if you get lucky, hey, you're even better off.)

You can't control what luck does for you, but you can definitely control what you do for yourself.

2. Make a lot fewer choices.

We all have a finite store of mental energy for exercising self-control.

The more choices we make during the day, the harder each one is on our brain--and the more we start to look for shortcuts. (When you're tired, you're a lot more likely to say, "Oh, the heck with it.")

Then we get impulsive. Then we get reckless. Then we make decisions we know we shouldn't make, but we just can't seem help ourselves.

In fact, we can't help ourselves: We've run out of the mental energy we need to make smart choices.

That's why the fewer choices we have to make, the smarter choices we can make when we do need to make a decision.

Say you want to drink more water and less soda. Easy: Keep three water bottles on your desk at all times. Then you won't need to go to the refrigerator and make a choice.

Or say you struggle to keep from constantly checking your email. Easy: Turn off all your alerts. Or shut down your email and open it only once an hour. Or take your mail program off your desktop and keep it on a laptop across the room. Make it hard to check, because then you're more likely not to.

Or say you want to make fewer impulse purchases. Easy: Keep your credit card in a drawer. Then you can't make an impulse buy. Or require two sign-offs for all purchases over a certain amount, because you will have to run those decisions by someone else (which probably means you'll think twice and won't even bother).

Choices are the enemy of mental toughness. So are ease and convenience. Think of decisions that require you to be mentally strong, and then take willpower totally out of the equation.

3. Put aside things you have no ability to impact.

Mental strength is like muscle strength--no one has an unlimited supply. So why waste your power on things you can't control?

For some people, it's politics. For others, it's family. For others, it's global warming. Whatever it is, you care, and you want others to care.

Fine. Do what you can do: Vote. Lend a listening ear. Recycle, and reduce your carbon footprint. Do what you can do. Be your own change--but don't try to make everyone else change.

(They won't.)

4. See the past as valuable training and nothing more.

The past is valuable. Learn from your mistakes. Learn from the mistakes of others.

Then let it go.

Easier said than done? It depends on your perspective. When something bad happens to you, see it as an opportunity to learn something you didn't know. When another person makes a mistake, don't just learn from it--see it as an opportunity to be kind, forgiving, and understanding.

The past is just training; it doesn't define you. Think about what went wrong but only in terms of how you will make sure that next time, you and the people around you will know how to make sure it goes right.

5. Actively celebrate the success of others.

Many people--I guarantee you know at least a few--see success as a zero-sum game. To them, there's only so much to go around, so if someone else shines, they think that diminishes the light from their star.

Resentment sucks up a massive amount of mental energy--energy better applied elsewhere.

When a friend does something awesome, that doesn't preclude you from doing something awesome. In fact, where success is concerned, birds of a feather tend to flock together--so draw your successful friends even closer.

Don't resent awesomeness. Create and celebrate awesomeness, wherever you find it, and in time you'll find even more of it in yourself.

6. Never allow yourself to complain. Or criticize.

Your words have power, especially over you. Whining about your problems always makes you feel worse, not better.

So if something is wrong, don't waste time complaining. Put that mental energy into making the situation better. (Unless you want to whine about it forever, eventually you'll have to make it better.)

So why waste time? Fix it now. Don't talk about what's wrong. Talk about how you'll make things better, even if that conversation is only with yourself.

And do the same with your friends or colleagues. Don't just serve as a shoulder they can cry on. Friends don't let friends whine; friends help friends make their lives better.

7. Don't try to impress others; impress yourself instead.

No one likes you for your clothes, your car, your possessions, your title, or your accomplishments. Those are all things. People may like your things--but that doesn't mean they like you.

(Sure, superficially they might seem to like you, but what's superficial is also insubstantial, and a relationship not based on substance is not a real relationship.)

Genuine relationships make you happier, and you'll only form genuine relationships when you stop trying to impress and start trying to just be yourself.

And you'll have a lot more mental energy to spend on the people who really do matter in your life.

8. Consistently review your long-term goals.

Say you want to build a bigger company; when you're mentally tired, it's easy to rationalize that you'll do your best tomorrow, not today. Say you want to lose weight; when you're mentally tired, it's easy to rationalize that you'll start changing your eating and exercise habits tomorrow, not today. Say you want to better engage with your employees; when you're mentally tired, it's easy to rationalize that you really need to work on some report today; tomorrow you'll worry about your employees.

Mental fatigue makes us take the easy way out--even though the easy way takes us the wrong way. That's why it's so important to maintain tangible reminders to pull you back from the impulse brink.

A friend has a copy of his bank note taped to his computer monitor as a constant reminder of an obligation he must meet. Another keeps a photo of himself on his refrigerator taken when he weighed 250 pounds so he's constantly reminded of the person he never wants to be again. Katheryn Winnick, the star of Vikings, keeps a list of goals on her computer desktop so she's forced to look at them every day. (She's also hosted a vision board party, one of the cooler ideas I've heard in a while.)

Think of moments when you are most likely to give in to impulses that take you further away from your long-term goals. Then use tangible reminders of those long-term goals to interrupt the impulse and keep you on track.

9. Count your blessings.

Take a second every night before you turn out the light and, in that moment, quit worrying about what you don't have. Quit worrying about what others have that you don't.

Think about what you do have. You have a lot to be thankful for. Feels pretty good, doesn't it?

Feeling better about yourself is the best way of all to recharge your mental batteries.