I use the same game shell for all my games - I guess this is a fairly common approach. The shell gives the intro screen menu, handles the high scores, game center, instructions and so on, and is just re-skinned for each game, meaning the only fresh code needs to be the actual game.  This has evolved over the last couple of years or so, but its still based on the original framework I developed for my first game, Stick Man vs Zombies.

My new game is the first that uses a physics engine - Box2D (http://box2d.org/).  There's a built in template in Cocos2D for Box2D, so I'm taking the oportunity to build a new framework, but with Box2D integrated.  This will also be based on Cocos2d 2.x rather 1.x (or 0.99x as it was originally).  It's a slightly time consuming process, although I am still reusing some old code, but I guess it will be worth it, as I'll have a new, up-to-date game shell at the end if.