Leave plenty of white-space. Crowded posters are intimidating and unappealing.
Consider using a sans-serif font for titles and headers (e.g. Myriad Pro), serif font for smaller text (the serifs make small text easier to read – e.g. Minion Pro). Font should be readable from several feet away, do not use small font sizes!
Try to refrain from transferring non-pdf files between different operating system. Missing symbols and incorrect spacing may result.