Magnets. Cut them small, take two attracting magnets glue one to the wall, and one to the corner of the poster. Do all four corners and the middle of the poster. You can even use chalk on the back of the poster pr-hanging to mark all the spots you would like to glue so everything is tight.
When you need to take them down, get a a razor blade, or xacto blade with lighter fluid. Use the lighter fluid on the wall glue. Let it sit for a few minutes. Then finely cut them away.
Only do this with posters you want for 2-4 years and not some collector piece you plan on hanging in your adult house someday.