Who
Where
Date
Time
Web address/email for more info

Clear Font
Not too busy a background - for those with hard sight problems
Great photo or image as the catcher
Don't clutter up with unneccessary info
Make sure there's a strong contrast with any font/text on images
Prioritise the most import info to be larger, least smaller etc
Remember, most people who look at a poster will only look at it in a few seconds - you need a catcher (a good image) to make people look at it for longer. Max to look at a poster is about 10seconds, after that if there's too much info then the viewer may get bored - or be moving anyway.
EWIS