If you are down and can't get motivated to work out, then I have the right trick for you. Watch videos!
Whenever I need a quick pick-me-up I usually go to youtube and browse. What I like about youtube is that the site contains all different kinds of movies. So if you don't get motivated by the movie I have posted above, then just browse and find what motivates you.
For me it's the video where people go through something they didn't know they could do. When they end up crying because they are feeling pain, relief and happiness at the same time. It's these emotions that are at the foundation of why I do triathlons. I know how it feels to be so proud of yourself and relieved that you're crying your eyes out, and these videos bring me back to the reason why I do what I do.
This is for me just a short fix though. It doesn't give me motivation to continue on for weeks, but it can help if I have problems getting out on a cold day. Give the movie above a shot, and tell me if you feel motivated!