What I've done in the past is get acid-free paper (scrapbooking type), and then acid-free, removable tape, put the poster on the paper, fit it to a large frame, and do it that way. Has worked fine for a variety of stuff I have (and yes, I've removed them and re-did it again, just to test it all).
But poster putty works too..but doesn't it leave some sort of stain mark?