I think a big mistake that people tend to make (and I've certainly done it myself) is that they wait until motivation comes FIRST before they take any action. Unfortunately you could be waiting for a long time with that strategy. What usually works better is taking action first, which then leads to motivation.

I'm not sure about you, but I always feel so much better after working out. So even if it's the last thing I want to do, I try to treat it like a priority and by the time I'm finished, I'm sweating like crazy and feeling great mentally.