I'm pretty slim fit too, and tried a few different knee/shin pads, most seemed to bulky and OTT, or for bigger legs. Got some Troy Lee ones now which are awesome. They have a neoprene type thing that you can leave on your knees and gives a good secure fit, then the guards attach to that and strap round your legs. Up shot is that they felt more secure than others (don't want the pad to move if you do crash, especially if you got skinny legs so they're looser), you can ride all day with the support type things that actually help your knees aswell, and you can stick the guards on in 5 secs. TBH you can wear the whole lot and forget about it pretty quickly though. They're not that cheap mind, but I got them half price in a sale at Skyline, so maybe have a look around.