The microswitch under the mouse buttons are usually the first things to go. Sometimes they can be fixed by cleaning or straightening the contact tab in the microswitch.
https://www.youtube.com/watch?v=82z-OpHdBgE
If that doesn't work and you're handy with a soldering iron, you can swap the left and right microswitches. The left mouse button is essential but most people can live without the right mouse button. Worst case, you can buy a new microswitch and solder it in yourself.