Rules For Writing Numbers by Jane Straus
The first rule for writing numbers, spell out single-digit whole numbers, sticks in my mind. I have no problem remembering it. On the other hand, I don't always follow the rule. Many times I alternate between using the numeral and writing the word, though not in one document. I do try to be consistent that much.
Beyond that, the rules require more concentration and recall than I care to or have time to put forth. It is then that I rely on my all time favorite, Grammar Book .com for help. Staying consistent within a category or writing fractions, large numbers, round numbers, decades, time, using hypens, and beginning sentences with numbers all require a trip to the rule book.
For a complete guide to writing numbers, make friends with Jane Straus and the THE BLUE BOOK of Grammar and Punctuation.