My friends laugh at me, my coworkers pity me, my family just will not discuss it with me, but I will defend the left-handed mouse technique ’till the day I die. Sure…when a mouse looked like the ones in this picture, it was alright to use your right hand. 
No one had figured out how a mouse was going to be used and most people were right handed, so it naturally followed that the right hand was used. I guess it is more correct to say that people used their dominant hand but less than 10% of the population is left handed. Due to the low percentage of lefties and the fact that they are probably already using the correct (left) hand, they will not be discussed here. For the other 90%+ people who use the incorrect (right) hand, WAKE UP! The only reason to use your right hand for your mouse from this point on is sheer laziness.
Ergonomics–
The ergonomics are better with the mouse on the left side of the keyboard. Take a look at the diagram below:
Left Handed Mouse Layout
As you can see from the layout, when the mouse is on the left side, it will be much closer to the center of your body. It will also be directly in front of your shoulder so that you can reach straight out to grab it. On the right side of the keyboard you have to reach way out past the number pad to use it. This moves your arm off center and increases stress on your shoulder.
Beyond the ergonomic aspects of using your mouse with your left hand, there are a few practical advantages as well.
Data Entry–
If you do data entry or if you work with spreadsheets often, you are able to input data with the number pad while navigating the sheets and clicking what ever buttons you need to simultaneously. No need to switch back and forth after entering data in each cell.
Multi-tasking–
Using your left hand to mouse with frees your right hand to type with, take notes with, or look through reference material with.
Here is a picture of my work station:
Dave's Computer Work Station
Notice symmetry and the notepad and writing utensils next to the number pad.
Some people may feel that it is to complicated to try to mouse with your left hand. This does not make any sense though. Think about a guitarist or pianist. both hands are used equally to accomplish complex movements. You type as well with your left hand as you do with your right hand, right? It will take about a week to get accustom to it, but after that you will never know the difference. I can switch seamlessly between using ether hand to mouse with at this point.
Some of you may argue that if you know the keyboard shortcuts you would not have to use your mouse at all while using whatever software you are using. True, but in that case you do not need a mouse at all so it does not matter anyway.
All that I am asking is that you give it a try for a week and make your judgment then.
