I've started and quit the gym four times in two years. What's actually different about people who stick with it?

It's almost never willpower, and almost always design. A few real differences: 1. People who stick with it pick something they don't dread. The "best" workout on paper that you hate doing loses every time to a decent workout you'll actually repeat. 2. They make the first step absurdly small at the start — showing up for ten minutes counts as a win, not a failure to do the full hour. Motivation follows consistency, not the other way around. 3. They attach the habit to something that already happens reliably — right after coffee, right after dropping the kids off — instead of relying on remembering or feeling like it. 4. They expect to miss days. The difference between a restart and a relapse is just whether you treat one missed day as data or as proof the whole thing failed. Four attempts isn't a character flaw, it's four data points about what hasn't worked yet. The fifth attempt just needs to be designed differently, not powered through harder.

It really all depends on the individual. A technique that works for some may not work for everyone who manages to commit to a daily exercise routine. That said, I can't speak for everyone so I'll tell you what works for me personally. Here goes: I typically start by day with a cup of coffee and listen to motivational music to help give me the edge I need to start moving. Coffee is deemed a stimulant that helps give me the fuel I need to follow through with important tasks from my to-do list which includes a morning workout.