No more than three fonts-- preferably two, one serif and one non-serif, with bold and italic of those two for variation.
No all-caps for any text you want people to read and remember-- it's the hardest type to read.
Avoid bold that is also all-caps-- that is really hard to read, and it shouts.
A little art gives life, but too much art gives headache.
A design that will work if xeroxed to half-size handbills gives you more options for less original layout cost/time.
If you want recipients to xerox and further spread it, use white paper and easy graphics to reproduce.
Size the type with viewing distance in mind. Readable from car to storefront? Or smaller, for hanging on a bulletin board people will stand in front of?
Leave plenty of white space-- it's the white space that makes the type WORK.
There are typography rules for proportion between lines of typoe as well as space between, but too hard to write. LOOK at good posters and see why they work, then use those principles.
Oh, and keep all the important stuff on the top third, so that if you mail it, and fold it for the envelope, all the grit is in the third they'll when they pull it out, and so there's no fold mark when it's posted that interrupts important info.