See the Willpower book for many specific suggestions. In general, self-control works like a muscle, and that includes getting stronger with regular exercise. Slowly make over your life with small improvements, starting with easy ones and working gradually up toward more difficult ones.

It's a continuum. Yes, some people have more than others, and also each person's powers of self-control fluctuate up and down. Again, though, it's more a continuum than discreet levels.

They would not necessarily have to be, but I think in practice there is some overlap. I would not expect a high correlation but it is something (and positive). When willpower has been depleted and self-control thereby impaired, IQ scores drop substantially, though mainly on the questions requiring effortful thinking (e.g., logical reasoning).

Self-control is one of the higher powers of the brain and hence more recently evolved. The brain evolved from back to front. So the front parts are most involved. I'm not a brain expert and am therefore reluctant to say much on this. But start with the very front (the pre-frontal cortex).

I wish I knew the answer from abundant data. Some brief studies have found that meditation restores powers of self-control when willpower has been depleted. Over the long run, meditation seems to have multiple beneficial effects. Insofar as meditation itself exercises self-control (such as by controlling one's mental attention), it should improve self-control.

Yes. It may not be as limited as we think, however. The body evolved to conserve energy, and it tends to conserve more than is necessary. Thus, the body acts as if willpower is quite limited, whereas in reality its limits are not so severe.

interesting question! I do not know that anyone has tested this. However, one can certainly fall into general habits of laziness.

ego depletion is a state of temporary low willpower. After you expend energy, the body seeks to conserve energy.

Food will work to some degree. Mind the glycemic index. For quick (but brief) effect, sugary snacks provide fast energy. For a longer period, protein is better.

Habits do not strengthen willpower. Nature enabled us to form habits because these are fairly automatic, which means they do not require willpower.

Yes. We find biggest effects with people who get less than five hours of sleep. Note that quality of sleep matters also. Nine hours of poor sleep will leave you with willpower problems.

Self-control and willpower help a little with dieting, but the effects are small. There are many things that affect dieting other than self-control. Plus, there is what the Willpower book called the 'catch-22' of dieting: not eating deprives your body of the energy needed for self-control.

Yes! Planning is generally helpful and adaptive, and planning how to use your limited willpower will enable you to get the most from this important resource.

We discuss that at some length in the Willpower book. Small habit-breaking exercises are a good place to start. Improve your posture by sitting and standing up straight. improve your speech (reducing curse words, favoring complete sentences, using full names instead of abbreviations). In some studies we have right-handed people switch to left hand for mundane tasks such as opening doors, brushing teeth, drinking from a cup.

Self-control can be used for starting an addiction, also for maintaining it (especially while keeping the rest of life in good shape). But probably it takes more self-control to quit than to start. People in addiction treatment or counseling usually want to stop, but if they want to maintain the addiction, they need self-control. For example, it is possible to smoke cigarettes regularly these days, but it takes planning and discipline, because smoking is prohibited in many places. Apart from that, I think the answer to the question is no.

This is quite difficult. Chronic pain leaves people feeling depleted (of willpower) all the time. I hope you are getting medical help with the pain. Beyond that, food and sleep are helpful.