Know how many colors you are using and what they are. Are you having this printed in CMYK, 1 spot color, 2 spot colors, black only, or something else? Make sure spot colors are set to spot, process colors are set to process, and remove all unused colors from the document.
RGB images? If your printer wants you to convert them (some do, some don’t), convert them to CMYK.
Rich black: If your document has large areas of plain black, consider using a rich black instead. Rich black has additional process colors in it. Ask your printer their preferred rich black configuration. If doing your own, try 60% cyan, 40% magenta, 40% yellow and 100% black. Keep in mind you don’t want to use any more ink than that -- this rich black adds up to 240% ink coverage, the maximum most printers will accept. Rich black can be played by ear. I often just use 40% cyan and 100% black.
Dot gain: Ask your printer what your maximum black should be. For instance, newspapers, which are printed at 65-85 linescreen, have a huge dot gain (how far ink will spread on the paper) and a maximum black recommended for them is around 75-80%. Why is that? In newspapers, 80% black will look as dark as 100% black. Uncoated papers will have more dot gain than uncoated papers. For most normal printing, you don’t have to worry much about dot gain, but it’s good to keep in mind for those occasions when you do.