What is the best way to develop a mind observation habit?
Well this is a suggestions question because your mileage may vary based on what you do and what's your environment. I'd suggest to use the most natural way available. 

The main idea here is to have a reminder in place. For this maybe you can use the entrances at your home or workplace to be a reminder. Or an action like drinking coffee or water or using the toilet. Or you can mark a person you see around you everyday. 

Side by side, cultivate a more natural inside reminder. Can't beat this one. This could be autosuggestion like telling yourself to be mindful and relax. Or it could be some action like scratching your head or ear. Or it could be any time you are walking. And you could use rubbing your knees when you're sitting. 
But you see all this requires predisposed awareness. Try to be aware of your body movements. This will certainly help in future.

In formal meditation, cultivate awareness of your sitting and the environment by using the sounds or smell or feel of the place. This would help in cultivating general environmental awareness in other places also. In the sense, this is more habit-forming.

These are some tips. But beware of the fact that any external thing you use, should not become a crutch such that when you are away from it, you lose all reminders altogether or you cannot be mindful at all.