A young lady grabbed our camera in a store and begged for help with hanging posters in her room. The scotch tape marks were just pissing off her mom too much.
Probably the best of the new gadgets for hanging things on a wall is the one that you see advertised so much by 3M, the plastic clip that is removed by pulling the glue tab out the bottom. It really does work, and comes off clean. Fun Tack can work, but will eventually get hard on the wall and can leave a bit of a mark and the FunTack imitations, especially the white stuff is useless. FunTack is not as bad as Scotch Tape, but not as good at the 3M clips.
You can minimise how many nails or clips you need by using splines to hang the posters. You can use the small ones that are sold for school reports, or find oversized ones at poster stores. Run a string through then clip on the poster and you just need one hanger per poster.