You need to have a trading plan that covers all permutations that the market can possibly throw at you. You need less willpower to follow a clearly defined plan than to try adhering to broad principles in reaction to the market.
Keep practicing applying your trading plan, so that you can make following the rules a habit. It is like driving, the more you do the less effort it requires progressively.
Motivate yourself, remind yourself of the importance of what you are doing
You need to remind yourself of the importance of achieving good trading results, of the importance of not throwing your hard-earned money away.
Use visualization techniques to picture situations where you follow your trading plan successfully. Thinking that you have lots of willpower actually makes it so.
Think of some trader you admire that have lots of self-control and unfazed by market movements (e.g. Ed Seykota)
Exercise your willpower
Willpower is like a muscle, the more you exert your willpower in whatever tasks, the greater your capacity for self-control.
You can get yourself to follow rules such as sitting up straight, opening doors with your left hand, etc.
Have sufficient food
Exercising willpower uses up glucose. Being hungry means you don’t have the energy to exert willpower.
Have sufficient rest
You can replenish your ‘willpower’ stores through sleep. Get sufficient sleep every day.