When the switches on my old mouse began to wear out and no longer responded to clicks, I decided to transplant the switches from an unused mouse to this one. The "donor" in this case was an old wireless mouse with a missing receiver.

Materials:
1 Unused "donor" computer mouse
1 "Recipient" computer mouse

Tools and Skills:
Basic Soldering
Multimeter
Screwdrivers

Step 1: The Transplant

First I disassembled the mice to free the circuit boards. These computer mice use a Normally closed (NC) switch that breaks an electrical connection when pressed, so they would still work for the recipient mouse. You may want to double check with a multimeter though.

Next, I desoldered the switches from both mice. In my case, the switches from the donor mouse were double pole switches. There was enough room for the right and middle mouse button, although I did need to bed the extra pin out of the way.
For the left mouse button, the sensor for the scroll wheel got in the way. To compensate for this, I replaced it with the one from the scroll wheel as it was not as worn and still working. Since I still wanted a working middle mouse, I replaced it with the other donor switch.

Here's a summary of what went where for my mice:
Donor switch -> Right mouse (Simple)
Middle mouse -> Left mouse (No room for DP switch)
Donor switch -> Middle mouse

After transplanting the switches, I reassembled the mice and tested it out. It works as expected, and I don't need to buy a new mouse!