It might not be the most obvious subject to be dealing with in a slots guide website but it is something that you should definitely keep in mind when playing online slots. Repeatedly clicking the start/stop button hundreds or thousands of times a day, sometimes with a bad posture or with the stresses of gambling in your body, RSI can creep up on you before you realise it.
Autoplay
Online slot machines should all have an autoplay function (sometimes prominent, sometimes hidden away in one of the smaller menus). Make use of it.
Relax
Sitting behind a computer in a stressed state is a major cause of RSI. Relax and keep your focus. Not only will this help prevent strains on your body but it can also help prevent you from making expensive silly decisions like chasing your losses too hard.
Take breaks
Again this is just good practice not only for normal computer usage but also for controlling your gambling.