The greatest motivation is to put yourself on "death ground" where failure is not an option. Sign up for something and commit. When retreat is not an option you will be motivated and active greater things that you would expect.
When you start to think about not working out visualize opponent getting stronger than you, beating you, and leaving you flat on your back in disgrace. Since you are committed already and paid the entry fee and your friends are expecting you to show up your only option is to push forward and you can't turn back.
Me for example I am going to sign up for and event called tough mudder ther is no retreat forme because I will have wasted my money if I don't attempt it. If I lose motivation I think of the possibility of failing, not completing the course and the regret that will follow. I cant let that hapen and my only acceptable option is completing the course.