Some of the larger office supply stores and catalog houses sell Blueprint storage racks designed for architects' offices, and these can make your posters take up much less room. Some work for rolled papers and others for straight hung papers; this last is the best choice for those with the room, but for more cramped quarters, the rolled-up storage can be done with the rack mounted high on the wall or even attached to the ceiling. Other racks such as hat racks might also be so adapted.
You don't want to fold or refold any posters, since with time they gain an antique value and could be quite valuable to you years from now. Folds only hasten the demise of a poster or any other paper, so try to keep them flat or at least rolled up inside a mailing tube (or similar blueprints tube) in regular rooms (not attics or basements where extremes of temperature and humidity are common). A mildewed poster will be of no value to you or anyone else. Likewise keep them away from sunlight or bright lights which will quickly fade and embrittle them. If a poster strikes your fancy or you think it will be a collectors' item in future, consider deacidifying it by use of a simple spray bottle of deacidifier available through the various Archival supplies dealers such as University Products or Conservation Resources, and others found by typing the subject into Google.com These places also sell the racks and tubes you may want to use.