There are a bunch of recipes in Annie Berthold Bond's book Better Basics for the Home. The one I use is a mixture of lemon juice, olive or jojoba oil, and essential oil of a citrus (lemon or sweet orange, usually). I usually eyeball it, but I think the recipe is 1/4 cup lemon juice to a couple of drops of oil and then about 10 drops of essential oil.
This is a recipe for wood that's not super-dirty, though. She's got recipes in the book for more hard-core cleaning jobs, if you can get your hands on a copy, or maybe search the care2.com website. Personally, though, if the wood is really filthy, I usually use Murphy's Oil Soap.