One of the clearest guidelines we have come across for poster layout is presented in a similar libguide composed at Thompson Rivers University Library. It recommends the adoption of the Rule of Thirds: "an image should be imagined as divided into nine equal parts by two equally spaced horizontal lines and two equally spaced vertical lines, and that important compositional elements should be placed along these lines or their intersections. Proponents of the technique claim that aligning a subject with these points creates more tension, energy and interest in the composition than simply centering the subject."
Do not overcrowd.
Image courtesy of Thompson Rivers University Library.