How to increase will power?

There are good answers here, but I believe they are missing something. Sometimes obstructions in cultivation (eg: lack of willpower) can be "fixed" by creating merit. So if you know the techniques to develop willpower (eg: meditation) but are unable to do so consistently I recommend you go to your local temple and volunteer regularly; most of the people who do this find that they become able to follow the instructions better.

By meditation. One of successful meditation method is "Anapanasathiya". You have to do is count your breath with full of concentration.

Meditation's pros and non-meditation's cons are required first.

Introductions of each meditation in the path of purification already guided whether how to grow the meditation inspiration by pros & cons vision, so you can see there.

For the example, 8fold path meditation, you have to see an introduction of sīla-chapter for sammāvacā sammākammanta and sammāācīva, then start to see an introduction of pathavīkasiṇa-chapter+ānāpānsaati-concentration-maditation-chapter for sammāvāyāma sammāsati and sammāsamādhi, then start to see an introduction of paññāniddesa-chapter to until the end of book for sammādiṭṭhi and sammāsaṅkappa.

Sammāvāyāma, in 8 fold path, is efforts to stop, keep stop unwholesome minds, grow, and keep grow wholesome minds. What is mind? sīla mind, concentration meditation mind, and insight meditation mind that is described in path of purification.

Human motivation (will power) is a product of karma (mostly the product of mental decision-making or mental actions performed in response to past experience) that extends back to one’s early childhood and to previous lifetimes. The result of this endless complexity is that what motivates one person does not motivate another. Given this fact of life, there remain two major sources of motivation that remain available. One is to be inspired or motivated by a genuine friend or teacher. The other one is based upon common-sense motivation based upon an explanation of how a meditation works that genuinely makes sense to you (not just to someone else, even a friend). The problem is that such an explanation is very hard to provide (by a Buddhist monk, guru, or psychologist). That is why most meditation teachers cannot provide an explanation except in terms of their personal experience, experience that may be very different from your experience. However, you do have a few options: (1) There are many schools of Buddhism that teach Buddhism in very different ways. Look around. You might find one that you like. (2) Recent developments have occurred in the use of mindfulness by psychotherapists. There are MBSR communities to be found in large cities. (3) You would probably do well to find a psychotherapist who you like and who can deal with suffering caused by a problematic family history. (4) If you are familiar with modern psychology and philosophy, you may consider reading my book on how mindfulness meditation works. You can download the ebook version for a few dollars.

Will power is very connected with virya - energy. Notice how, when you feel inspired, what they call "on a roll" - you can do things that are normally difficult, easily applying force as needed. In the same vein, when you are depressed and demotivated, doing even the simplest chores requires lots of effort to get going.

So the secret to will power is to develop the faculty of energy (viriyindriya).

Development of energy is not directly emphasized in Pali Suttas but in some Vajrayana schools it is the main practice, esp. in higher tantras.

So, how does one develop energy? In real teacher-student situation, you learn it by watching and imitating the teacher, and from occasional very casual and often sarcastic pithy instructions. To cover all that in a single answer would be impossible, but here is a brief summary of what I was taught:

• Vitakka / vicara aka autosuggestion to raise inspiration, e.g. what football/basketball coaches do. It looks silly but it works, esp. if you connect it with emotion.
• Wise application of attention - avoid dwelling on "victim-perspective" topics that make you feel hopeless, focus on things you are mastering / have mastered.
• Discontinue or minimize activities known to sack energy, e.g. alcohol, masturbation, facebook, arguments.
• Mindfulness of the body and overall grace. Learn to always feel comfortable: walk comfortably, sit in comfortable posture, stretch enough, move gracefully etc.
• Mental comfort / speed - do not rush yourself, do not beat yourself, give yourself enough respect.
• Take responsibility and learn to live on your terms.
• Stop seeking outside stimuli (so-called "hunting dog mind"), slow down and find the right practice right where you are, in your everyday activities.
• Use Meditation as another chance to stop and get to your senses, as opposed to forcing yourself into something contrived.
• Develop stronger core muscles, esp. abdomen.

Out of all these practices, the principal one, and what I will emphasize the most is living on your terms. Maybe because it was the most difficult and eye-opening to me when I learned it.

What weak-willed people do a lot is allowing circumstances or people to rush us or to force us do things we are not Okay with. Instead, we should learn to take responsibility for our lives, take a stance - and methodically live our life on our terms, step after step. If you start on a solid foundation, every subsequent action will build a solid step, and another one and another one. Do not let anyone ever force yourself go against your best judgement, nor go against your judgement because of urgency or time pressure. Do things impeccably to your own standards, look impeccable in your own eyes - and from this will come energy and strong will power.