I always teach my students these things to consider when coming up with posters:
STOP
LOOK
LISTEN
Their poster must have these 3 ingredients.
STOP- it has to make their TARGET AUDIENCE 'STOP' and look at their
posters because of its design. It has to be attractive.
LOOK- now that as an audience i am DRAWN to your poster, i will stop
and LOOK at the elements on your posters, TEXT, IMAGES, ETC. they have
to be unified and cohesive.
LISTEN- Whatever MESSAGE the poster is sending out to the AUDIENCE has
to stick with them. It needs to have an IMPACT. Poster needs to GET
the MESSAGE ACROSS.
then...
SOMETIMES, what I do is i give them templates, example, divide the
poster into three parts, one for image, one for text, one for contact
info... i also make them look at magazines, and loosely base their
lay-out on some of the exisiting ads. The internet is also a great
source for examples and pegs.