Too many posters suffer from trying to do too much. I suggest two rules of two to limit the damage.
Two typefaces. That gives you one typeface for the bulk of the poster text, and one display typeface for section headings. The only situation I can think of where you might need to use other typefaces is if you have some sort of foreign language or special symbols in your text. And don’t you dare cheat and say, “Well, Arial and Verdana are both sans serif fonts, so I’ll just count them as one.” The variation between regular, bold, and italic don’t count towards your limit, though.
Two colours other than black and white. Use those two colours as your artistic little splash for backgrounds, to fill in bar graphs, colour your text if you want. You can vary their intensity, saturation, or what have you, but having large swathes of poster in three or more colours is rarely going to do you any favours.
But break either of these rules rather than do anything barbarous.
Photo by qthomasbower on Flickr, used under a Creative Commons license.