1. Document your sources fully. When you quote from a book, magazine, or other print or online source, provide a citation with complete publishing information. If you don't, and if I have to research that information myself in order to show it properly in your endnotes, the time I spend will cost you.
What does complete information look like? Consult this resource (courtesy of The Chicago Manual of Style).
2. Avoid using quote compilations. Whether they're online or in print, collections of quotes are not dependable sources. They work great for speaking but not for writing, which requires thorough and accurate documentation. When you use trustworthy sources (e.g., you quote Twain directly from, say, Huckleberry Finn, not Goodreads), you make my job easier and cut your costs accordingly.