Do always keep  the mouse on a convenient distances from your body, and hold the hand relaxed. 
Keep the wrist so that you be soft like a cat in the hand.
Learn how to use to use all the shortcommands on the keyboard, then you don´t have to use the mouse equally often, one can accomplish many of the most common commands with different
keyboardcombinations. 
You can use the up- and downarrows in order to utilising the scroll-list. 
Make sure that you have a properly support for the wrist, if you know that you have an uncomfortable angle between the arm and the hand. 

A hint well worth to try have I received from Daniel, thank you:
" I have a  hint for them who suffers from musarm/bad wrists. 
Instead of keeping the mousepad on the right side (if you are right-handed), you can keep it to the left in front of the keyboard.
Then you can put the entire arm in front of the keyboard. In that way your left arm is almost entirely relaxed. 
It takes a while to get used to it, but you will get used to it. Set the mouse on high sensitivity in the control panel."

Exercise 1. 
Doe a fist. Hold left in 10 seconds. Spread with the fingers. Hold left in 15 seconds. Repeat 5 times. Relax. 

Exercise 2. 
Hold your arms straight in front of the body, intersect right over left and let the palms meet. Entwine the fingers as that you flicks the hands. Hold left in 15 seconds. Pull the arms against the body and turned up your fists in front of the breast. Hold left in 15 seconds.
Do the same exercise with left arm over right. 

Exercise 3. 
Press together the palms in front of the body with the fingers pointing upwards in an angle between upper side of your hands and forearm (sanctimonious prayer). Hold for 15 seconds. 
Turn the hands so that the fingers points downwardly. Hold still with palms pressed against each other. Hold left in 15 seconds. Press the upper sides of your hands against each other with the fingers pointed downwardly. Hold for 15 seconds