The hunt for the perfect Pad Thai recipe

Seeing as I've never been to Thailand, I don't really know what the perfect Pad Thai would taste like. However, I have had some fabulous Pad Thai in a few Thai restaurants, and I've since been trying to replicate it at home. I attempted it again tonight for dinner, and failed. Not miserably, mind you, but enough to make me wonder about what exactly makes great Pad Thai so great. Do I really need to use ketchup, as many recipes (none of which I've tried) suggest? Is there some magical Thai ingredient that I'm not using? Am I just never destined to make the perfect Pad Thai in my own kitchen? Does anyone have any suggestions? -K.