If the switch seems dodgy, take off the back cap, and you can see two little holes in the switch endy bit that let you unscrew it. Unscrew it a little, then screw it back up tight and make sure it is quite nice and snug.
You need something to stick in the holes, obviously a pin spanner or something made for the job is perfect, but if you haven't got one, use something random and pointy – I used the leads from a multi-meter, cos they were what I happened to have handy. If it is a little loose, you get weirdy stuff happening with the button – I know this from experience thanks to my urge to take things apart and my lack of care when putting it back together.
Joe