Design what you need right now.
Most likely, what you need right now — in order to try out your game concept with some friends tomorrow night — isn’t a fully written, well thought-out game or even a full playtest draft. Most likely, what you need is a bunch of notes, maybe notes only you understand, hacked together from some of your better ideas and some other stuff blatantly stolen or hacked together from other games.
Along with that, play early and often.
When you are stuck, when you are tired of writing or think you might be heading down the wrong road, when you’ve spent more than a week without making real progress on a project, it’s time to take whatever you have and play it. The sooner, the better.
Design for yourself (and your friends) before you design for others.
What do you want to play tomorrow night? Design that. Publishing… what’s that? Don’t even consider that until you have a game that you want to play with your friends tomorrow night.
These are all thoughts directed at myself. I am the “you” mentioned above. But if other people find these useful, that’s awesome too.