Step 2: Give yourself props for progress
How many times have you said to yourself during a workout, "I'm getting nowhere"? Nothing evaporates motivation faster than feeling like you're not making any noticeable improvement.

The problem: When it comes to working out, women are notorious for seeking a comfort zone. Once we master a new skill (like holding plank position for 60 seconds or running at a 10-minute mile pace), we stick with it because, hey, we know we can do it. But it also impedes progress and breeds big-time boredom.

"The less interesting something is, the less motivated we are," Wilson says. Some of his earliest SDT studies showed that humans have a basic need to feel engaged—take away the novelty, and motivation vanishes. And Groundhog Day-style monotony isn't just bad for your head; eventually your muscles stop responding and you really hit a wall.

The solutions: Mixing things up and pushing yourself. "Changing the intensity and type of exercise trains the muscles differently, and you'll start to see improvements more quickly," Wilson says. For example, increase the weight you're lifting and the number of reps and sets by 10 percent every week. The same goes for your cardio—increase the amount in 10 percent increments each week. Do this for three weeks, and then drop back down to where you started on week four to let your body rest, Cosgrove says.

Next, write everything down. A workout log functions not only as an exercise checklist but as a concrete record of how far you've come—a way to motivate yourself if you become frustrated. In researcher-speak, this is called establishing competence, and it's at the core of the second step in fueling motivation that lasts. To make it work, keep the focus on what you can do, rather than what you can't, Wilson says. And don't compare yourself to anyone else. Once you start focusing on you, your confidence will grow and ignite a cycle of positive reinforcement that will keep you hooked.