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.