Bending your wrists! It causes irreversible Carpal Tunnel, ie. it makes bass playing extremely painful...
to fix the fretting hand look at mysticmichael's post
bad habits with the picking hand....
most players, while in the sitting position and sometimes while standing, use the top of their bass to lay their forearm upon. That is fine except when the position of your elbow is too low and your wrist naturally bends. Also while moving up and down the strings they simply bend their wrist.
If you find yourself doing any of this I suggest you do the following which I'm still doing myself... (I "relapsed" back into my bad habits haha)
Get in front of a mirror with your bass (do both sitting and standing)
play any tune with your natural posture and composure
only observe your hands, do not try and change anything about your playing yet
note specifically what you need to change
now to fix it... adjust your elbow and the bending of your arm until your wrist is straight yet comfortable and keep it straight while cross stringing
it may seem too troublesome at first but it works after enough time is put into it and your wrists will thank you!
also cracking joints isn't considered "harmful" but still try and avoid it.