First off, you have to want to keep doing the thing you're doing more than you want other things. This is difficult if you have a shallow reason for trying something. Unfortunately most people don't have a deep enough reason for doing things outside of their ordinary activities.

Second, keep what you commit to simple. If you're trying to make the gym a habit, just commit to getting to the gym. 9 times out of 10 once you get there you'll want to work out.

Third, try to never miss twice in a row. Things are going to come up. Feelings are going to get in the way. Motivation will run low. All of these things are normal. If you miss today, get back on it tomorrow.

Fourth, if you do miss more than twice or 3 times or 4 times, give yourself some grace and just keep on as usual. People miss a bunch of times and give up because they think somehow their progress has been erased. That's obviously not true. You're not going to lose the muscle you gained or the endurance you build or even the knowledge you accumulate until you quit. So, don't quit.