The best thing to do is frame them; you can get readymade poster frames at chain frame stores. If you like the unframed look, you can also have a frame shop put a backing like black or white foam core on them. Or if you just want to stick them up as they are, you can try removable poster putty on each corner (and midway along the edges for large posters).

i found these things called Magnetacks that are magnetic poster hangers that are magnetic tacks that hang posters without damaging the poster. they are strong so they work really great for unframed posters and photos or whatever. check them out here: magnetacks link