Determine your audience. Are you writing a book for young children, or is it geared toward an older audience? Do you intend for people outside of your home country to read your writings, or is it unlikely to be read by anyone outside of your own town? Is there a specific gender the book is written for? How about a specific cultural group?
Make it age appropriate. Your writing style must be well suited to the knowledge, abilities and sensibilities of the age group you are writing for. Complicated metaphors will go over the heads of most children, while the simple sentence structure that would be suitable for kids would bore most adults to tears. No big or profane words for children, either.
Make it geographically encompassing of your audience. Just because you and your neighbors use "Coke" as a generic term for all soft drinks doesn't mean that someone in California will get the reference. This becomes even trickier when writing for an international audience; there's no internationally agreed-upon term for the storage space in the back of a car, for instance. If you're really stumped at what the best word to use is, try using an authoritative source such as the Oxford English Dictionary for help.
Make it culturally accessible. Avoid ethnocentrism (the assumption that one's own culture is the standard to which all other cultures ought to be held), as it can needlessly alienate readers from different backgrounds. Don't go to the other extreme and specifically address each cultural group that could possibly read your writing either (if writing a tourist guide for Jackson, Mississippi to be sold in local bookshops, you need not specifically mention the best place for natives of Papua New Guinea to buy phone cards to call home). Use common sense -- assume that anyone with the necessary education and maturity to read your material could come from countless backgrounds, and do your best to be inclusive within reasonable bounds to those people. Don't use controversial cultural terms, either, particularly where race is concerned.