Determine why you're doing what you're doing.
I firmly believe that you have to be interested, passionate, or receiving a reward from something in order to be motivated.
Passion and a reward can also lead to discipline, the glue that holds you together as the waves of motivation come and go.
The only way to find your passion is to try as many things as possible; fail hard, fail fast, learn, and move onto the next.
During this process you have to take a step back and ask yourself why each pursuit is of any meaning or purpose to you.
Find "that thing" that provides you with fulfilment, and motivation will follow you like a shadow.
When it comes to being rewarded, once again, take a step back and understanding that everything you're investing your time and energy into now will allow you to get to where you want to be... should you have crystal clear vision and take action each and every day.
A reward doesn't have to be tangible (money, a treat, a night off), it can also be the light at the end of the tunnel that keep you pushing forward.
And that starts with determining why you're doing what you're doing.