how do you prevent hand injuries due to continuous handling of mouse ?

As Joules said, just take your hand off it for a while :-D

Have a workshop to play in so you take regular breaks from the computer. The exercise of moving is good for you too....

I had same problem, but by using this:
http://www.kensington.com/kensington/en/gb/p/1429/62404/memory-gel-mousepad.aspx
You have to use a while before getting better.
Now I feel much better...Not perfect but much better

Make sure that your hand runs parallel to the table you are sitting on by setting your armrests correctly. This will help prevent your wrist resting on the table and putting too much weight on it. For most tables this means you might need a stand to raise your monitor or laptop to eye level, as you also want your legs parallel to the floor. Most people set their chairs too low for this, which can cause knee problems.

You need an L shaped desk where a leg of the L is at your mouse side. You rest your complete forearm from elbow to hand on the desk. Like an armrest sort of but part of the desk. This takes all the force/weight of your arm off of your wrist and hand. You can also pad the desk where your arm rests with an oversize mousepad.

Height of chair and desk is important also. Your feet should be flat on the floor and arm comfortable resting on the desk. If your chair/desk combo is too high to have your feet flat on the floor, or if it feels better with your feet up a bit, use a small foot stool.

Duct tape and 3 helium filled balloons . .it will keep your wrist elevated. For any extreme pain, take two Vicodin. If pain persists, see a doctor.

Even if i set my position properly, i get pain in my wrist...but the Mousepad help a lot...

I have an "L" shaped desk, and my keyboard and monitors are situated in the corner, so my entire forearm is able to rest on the desk.
I also have my mouse speed set really high. My wrist hardly has to move...
I've tried some of the trackball mice, and just couldn't get used to them.
I'm pretty efficient with a standard mouse, and my highly customizes "S" key.
The jobs I've had throughout my career have always involved more than just being a "CAD operator", though, so I get up and go out on to the shop floor to look over my designs, troubleshoot, and see where assembly problems arise, or where we can improve the designs.
Occasionally, if I'm at my desk for a long period, really deep into a design, I'll have to "pop" my wrist, and when that happens, I get up and go for a walk and stretch my arms and wrist out.
In 30 years of CAD design, I've never had any trouble with my wrists.