ISiskills directs you to a couple of courses and leaflet about poster creation. Search for poster.
Books exist in the library on scientific writing and communication and include poster creation. Some classmark areas which would be good places to browse:
QA1778, Q222, Q223, T11 [PE1408]
See also Flora Grabowska's, a librarian at US's Vassar College:
list of poster design sites
look to the left menu