Long projects have a way of getting away from you. It’s hard to keep track of everything you’ve done, and it’s especially difficult to remember why you did it. Those times are the reason to keep a changelog.
You might have heard of a changelog in the context of programming. If you haven’t, it’s just a list of what you did for each version of your game/program/whatever. “Version 3–Reduced player chits from three to two, increased food to five units/player” is a perfectly good changelog entry.
Just doing that much will be a big help to your design. If nothing else, it will help you reverse course when something goes wrong (and I guarantee you, things will go wrong). A summary of what changed from version to version makes it easy to undo changes and get back to a version of the game that worked.
To get even more value out of your changelog, include a brief statement of why you did it. “Reduced player chits from three to two to avoid stalemates” is a short statement, but it allows you to reverse course selectively. If you’re still having a problem with stalemates, you can undo just the change that was meant to deal with them and try something else. If you’re happy with the stalemate situation but something else is wrong, you can roll back everything else.
Adding that extra detail to your changelog will also keep you from making stupid mistakes (of a sort, I will admit, I’ve made 😉 ). It’s very easy to forget which changes responded to which problems. You can very easily undo or modify a change that you need to keep, simply because you’ve forgotten what it was doing for you. Being able to quickly reference the changelog and say “wait, this thing that’s about to be altered is doing important work” can save a lot of time going down blind alleys.
For an example of a good changelog for a tabletop game, check out the one Mantic Games just put up for its Warpath rules. Most changes include a specific statement of why the change was made, so it’s clear which ones are safe to fiddle with and which ones aren’t.
(Disclaimer: I’m a backer for the current Warpath kickstarter, but only for about $20 to get a couple of specific models to use for other games. I don’t need more stretch goals met. ;))
Keeping this sort of changelog is the kind of thing that will take a few minutes each design/playtest session, and will save you hours of completely unnecessary design/playtest sessions. I can’t recommend it enough.