1. Start with stripes. If you've never mixed print before, try something simple like a striped cardigan over a striped top.
2. Break it up. Another good place to start is print shoes or a bag with one other print piece. The neutral space between the prints will keep the eye from being confused.
3. Common colors. Whenever I mix prints, I wear things with the same background color. My accessories will be other colors from the print, tying it all together.