I have the luxury of building two web apps for the business I run. One is fairly simple, which is nice for easing me into Rails. The other, though, is pretty complex. I decided to start building bits and pieces of it, then when I get competent enough with Rails I can either correct my mess or completely rebuild, which won't take long considering the foundation of the app will already be in place.
Because I'm finding my feet, my desk becomes rather messy with reference books and notes laying about. It's especially handy to have my Moleskine beside me at all times; I find noting down the steps I've taken to build the app helps when it comes to returning to it after a week or so of other work.