I find my mice break a lot, too. Generally what happens is the left button starts to kill itself. The last time this happened I had a spare laying around so I just replaced it, but the spare's fucked itself over now. I switched the buttons so that I could keep using it without fucking with hardware, and in games that require use of both mouse buttons a lot I have one mapped to middle click. Neither button is completely dead, but left click always double clicks instead of single clicks. I didn't bother remapping it in TF2 because the only character I need alt-click as(atleast, pressing it!) is Pyro, and airblast's fire rate prevents the problem from affecting me.
Of course, holding the left mouse button is faulty, too. I often end up spinning down my minigun as the Heavy when I don't want to, which is a major issue since I usually use the Brass Beast. Luckily, it doesn't happen all that often.