I buy a $1 project notebook, put the name of the project on the front page, and start jotting down things in the notebook. This could be database tables, paper mockups, notes on features to add, roadmap, whatever.
Eventually, on a day I'm feeling particularly inspired, I can't stop myself from coding. This sees me make a quickie Rails development environment and push hard to get One Cool Thing accomplished. As long as I have one thing done, further attempts at coding don't feel like breaking ground (hard work!), they're just extensions to existing functional code.
You'll laugh, but the One Cool Thing I did with Appointment Reminder was making it so the logged in user can switch the color scheme for her pages. This has rather little practical utility but it was quick to do and satisfied my monkey brain: "visual changes == making progress, ergo, five visual changes == LOTS of progress."
Often the first thing I do is something with domain logic that I can only execute through the console to start out with. Crikey I love my Rails console. Right now for client work I have a program which goes to a particular government agency, slurps down a gigabyte of data, does some number crunching, and then tells you things any well-informed high schooler knows about a particular subject. The difference between the program and the well-informed high schooler is that the high schooler would get bored after listing 5 or 10 of the facts but the program can punch through thousands.