Like physical strength or stamina, we can build our willpower like a muscle, through regular practice, I have observed.
When seeking to build willpower its best to understand that it is the ability to take challenges that you don’t want to do. The easiest way I have found to practice my willpower is to choose activities for me to do. I hated swimming, “well I still do” but I chose to increase my willpower by installing a regime of doing a minimum 500m swim (10 laps) at the local pool 5 days a week. I planned in advance how to deal with the several excuses, mental gymnastics and physical struggles I would expect to endure. It is amazing the amount of willpower you will gain by combatting the temptation to stop.
It is always hard at the beginning, but it gets easier.
Another goal might be to do 100 sit-ups a day for example. What you will notice is that as your strength and ability grows, you can take on more challenges and increase your “Willpower” Workout.
I now do 30 laps to 60 laps 5 days a week, but it isn’t the extra km’s and cardiovascular ability that has had the greatest affect, it is the ability to take on more challenging goals and to follow through.