We all have them, so let's gather around and confess. I'll start..
Instead of using F5 to refresh, I do so by pressing CTRL + R. Not only that, but I browse with my left hand fixed on the CTRL + R keys. This places unnecessary strain on my hand.
Swap Caps with Control, and that strain goes away. It's miraculous.
Sitting too close, so that my arms are making a / \ shape
That's actually not that bad, as long as you're wrists are straight. It's the unnatural wrist position that puts the most strain on you while typing (typically). My forearms are usually at close to a 45 degree angle when typing.
As far as my bad habits go, on a standard keyboard layout I hit the backspace key with my ring finger, and only use my right thumb for typing. Otherwise I'm pretty much picture-perfect.