How to Stay "Motivated"

Just to be clear I'm not a fan of "motivation" to help someone stick to whatever activity it is that they want to stick to.  Motivation can be OK to get you started but not really the best at getting you going when things get difficult.  Discipline and having an overall view of why you're doing something will always be better than looking at motivational quotes and videos on social media.

That said, for those who really want to stay "motivated", I would say that the best way is to constantly look for what you're not good at and finding ways to improve it.  Set an achievable goal and work on that weakness.

Let's take martial arts as an example.  If you think that you've gotten good at punches, make a goal to make your kicks better.  From that goal you can pick an aspect of kicking that you can improve.  You can try to improve your hip mobility and leg flexibility to kick higher; or maybe try to improve the speed of your kicks; or develop the reaction time to use your kicks as intercepting counter-attacks; or make the accuracy of your kicks better.

You get the idea.  The point is, be honest with yourself and find out what your weaknesses are. Then set a goal to improve on those weaknesses by breaking them up in smaller sub-goals.  Completing those smaller tasks will help you see that you're progressing and the pursuit of those "small wins" can be very helpful in keeping someone motivated.