Edit: Here is a image which isn's as dark:
This wire controlls the notches that you feel when you scroll. Remove the wire and the scroll wheel will be free-rolling. The wire can also be bent a little to provide stronger or weaker tactile bumps at the notches.
This wire controls the force needed to press the scroll wheel down. I like to adjust this one since I find that some models have a scroll which is to easy to push down and I accidentally middle-click when I want to scroll.
This is the shell of the mice as viewed from the inside. The marked stem-looking thing that portrudes the shell is what pushes on the micro switch located on the PCB inside the mouse.
You can glue small pieces of plastic or whatever you have around, credit cards work great when the mouse has lost its crispyness. Credit cards are however in most cases too thick. Some mice you have to press the button far down before it actually even reaches the micro switch. This alleviates that. Add enough material and it will however be far to easy to press or be constantly depressed. Just remove some material if that happens and try again. I usually cover the small piece I glued there with glue since the surface becomes a bit softer. This means that there is less of a rattly sound when pressing the mouse buttons. I like this one in particular because it makes the buttons more crisp, easier to press, less travel distance and less rattly.
The micro switches can be bought at a lot of places in case you want to replace one, broke one, want different hardness etc. Note that they need to have the full shell and not have an exposed spring leaf. Here is a store which sells them ans ships worldwide for instance.
http://www.dealextreme.com/p/diy-mouse- ... 495?item=2
Edit: I forgot to write that warranties will probably be voided once you even open your product, let alone modifies it yadda yadda yadda I take no responsibility if you mess it up blah blah blah disclaimer etc.