Building willpower is similar to building muscle. Continually exercising without giving yourself a break is not the best way to increase your strength or performance. 
Don't allow yourself to be in a position where you constantly have to exercise self-control. It will eventually deplete your resolve. Allow yourself some recovery time away from temptation.

Avoid willpower depletion

People with high willpower use it to avoid getting themselves into a crisis.
If you can’t prevent temptation, make a plan in advance for what you are going to do instead of succumbing.

Have a plan

It takes an immense amount of effort to suppress your normal personality, preferences, and behaviors. 
Hold true to your own internal goals and desires.

Be yourself

When people are stressed, they tend to fall back on ingrained habits - regardless if they are helpful or harmful. Start responding to mild stressors with better choices. The more you strengthen your habits, the more likely they will be there to rescue you when a major stressor comes along.

Build good habits

People give up because they feel overwhelmed by the enormity of the goal they must accomplish. 
Break the goal down into manageable pieces and line them up in a sequence that guarantees that you will never put yourself in a state of willpower depletion.

One step at a time

Imagination can blunt the cravings that erode your self-control.
If you imagine lying on a peaceful beach, your body will respond by relaxing. If you imagine being late for an important meeting, your body will tense in response. Use this to your advantage in building willpower.

Use your imagination

You can even use your imagination to keep unwanted thoughts away. 
Every time that unwanted thought occupies your mind, consciously think about something pleasant instead.

Think about something else

A growing body of evidence indicates that willpower and self-control are essential for a happy and successful life. The most persuasive evidence comes from two studies that measured young children's self-control, and then kept track of them as they grew into adults. The most well-known experiment, the " marshmallow experiment," began in the 1960s by psychologist Walter Mischel.

Think about something else

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Treating Thoughts Like TV Channels

Let your mind wander without judging your ideas.