Have absolutely clear idea about what you are doing.
This works in so many level. In higher perspective it’s a definite thing needed to finish a game. For example, having a solid answer to “what is this game all about?” is a must (I was working on certain feature today, and my game’s core values helped me decide how to approach that feature). There must be direction.
In lower level this works as well: two days ago I had a really clear and good specs for my work (and I finished good amount of stuff, was really pleased with the results). Yesterday… I had a pretty good day, but not as good as the day before. I must admit that some of the yesterday’s tasks were bit blurry (to some extent), and that’s probably the reason why I didn’t finish the things that I planned (and actually ended up working couple of hours on some feature that I had delayed for “to do in future versions”).
Do you know what you are doing today? Do you know what you are going to do next year?