A hot glue gun loaded with low-temp glue sticks works wonders attaching posters to the walls of my classroom. The glue comes off the back of laminated posters very well and doesn't stick and smear like that clay so many educational catalogues stock. It comes off my walls, both brick and stucco, both painted and unpainted, without peeling or smearing. And best of all, it doesn't leave holes in either the walls or the posters. A couple of hints: check the spot on very old paint to see if the glue pulls the paint away from the wall. My walls are painted latex semi-gloss and it does not peel. For unlaminated posters, reinforce the corners with a square of book tape so that the glue doesn't adhere to the paper directly.