What do you use to stick poster up?

I used blue tack to put up posters in my disabled son's room. They do not mark the wall. I bought a poster, trimmed it and put it in a black glass fronted frame. I put it up in my lounge. It is on a nail with a raw plug.

They make some stuff that looks like clay that works fairly well, if your walls don't ever get too warm or too old. It's called Sticky Tack, I think. The problem I've had with it is if you leave it in one place for too long it will eventually make a grease stain :( On the wall and the poster. I'd never heard of using toothpaste. I've heard of filling holes left by tacks or nails with toothpaste but never to use it as an adhesive.