This works out well for kids who are always adding more or rearranging things or for prints you know will just be a phase. Obviously, art prints should always be framed or hung from clips to be protected, but when it's time for Taylor Swift, toothpaste (or shampoo) it is!
A few things to note: You should not use a whitening toothpaste for this or something with "things in it." You know toothpaste these days, it's always packed with breath strips or something wild. Stick to the basics and you'll be all set. Shampoo works best when it's not "hydrating" which won't have the same sticking power.
Both will allow the poster to peel right off, assuming it isn't left on the wall for more than a few years (think less than 5 to be safe). It doesn't take much and you won't have to rummage through the garage to find a nail, tack or something else.