Paper prototyping is an essential step for every design process I start. On the internet you can find a remarkable number of kits to help you work better, such as the precious notepads made by UXPin, that go from a simple smartphone/web browser wireframe to a series of ready-to-use printed elements that you can position on your paper prototype.
When paper prototyping, I usually prefer to take all the advantages that paper, pencil and rubber can give to me, that’s why I leave pre-rendered elements for the digital mockups phase.
I find sneakpeekit sketch sheets extremely useful for paper prototyping: they are minimal tablet/smartphone or web mockups, with space for notes and project details. You can choose among a “quick sketch” version (more than one mockup per page, useful for fast drawing) and a “detailed sketch” version (with grids). sneakpeekit sketch sheets are free to download.