Cap your time

Here's an idea. Break your project into phases. The first phase is something playable with a couple levels; the second phase adds a bit more and has all the levels, story, bosses, and music implemented; and the third phase is for polishing your game and filling in the gaps.

Now break each phase down into hourly estimates of categories. For instance: planning and design, programming, artwork, level design, music and sound. Here's an example for a small game:

Phase I
Planning and design: 2 hours
Programming: 15 hours
Artwork: 10 hours
Level Design: 10 hours
Music and sound: 10 hours

Phase II
Planning and design: 3 hours
Programming: 30 hours
Artwork: 20 hours
Level Design: 30 hours
Music and sound: 15 hours

Phase III
Planning and design: n/a
Programming/bug-fixes: 10 hours
Artwork touch-up: 5 hours
Level Design fixes/changes: 10 hours
Music and sound touch-up: 5 hours

175 hours! Initially I was thinking it'd end up around 50 hours, but breaking it up into phases showed me that I greatly underestimated that. And when I estimate my hours I like to overestimate, because things aren't going to go as smooth as you think they will half the time. Breaking your game up into phases does 2 things: first, you focus on reaching the end of the phase and not the end of the game's development (so you're less likely to be caught up with feature creep and the like), second your hours are better estimated because you are estimating hours with more concrete markers than just "finishing the game."

Here's the second part of my idea: DO NOT GO OVER YOUR HOURS!!!

Why? Because if you go over the when will it end? Ideas and the quest for perfection always get me spending more time on little details than I should. Also, you if keep to your word, when the time gets close and you're not quite there you will have to finish up with what you have, even if that means putting in plain (placeholder) graphics and just the raw bones of your code. The goal is to reach the end of the phase, not to complete the game. And once you reach the end of the phase, you can assess where you are at and where you want to go and plan your next phase. This type of planning/execution/planning pattern is used in commercial software companies in one form or another; some examples are Scrum and Agile development. There's power in this strategy as it keeps you focused on more realistic (and closer) goals. It also allows for changes to be made if needed (re-planning a phase, for example).