Click the book at the bottom of your screen and look at the relation log. it will tell you everything you need to do for every heart on all characters. If you missed certain steps, you'll need to go back before you missed your chance or, restart.
Of course there is the ultimate way of cheating, that being downloading the Universal Ren'Py Mod. This allows full access to all games made with this engine. You can manipulate practically everything! Be careful though. If you don't know what you're doing, you may screw things up and you'll have to start from scratch.
Have fun!