The only way I can think of is to use a frame ;)
You don't need to frame each poster, but create something like the advertising hoardings you see on bus stops (in the UK at least) where the frame can open and a new poster inserted.
So what you'll need to do is get a piece of glass or Perspex slightly larger than the poster you want to hang. Then create a frame with a cross section something like this:
| space for poster |
|------------------| <- frame
+- ^ -+
If you have this on three sides the poster can slot in at the top. It will rest against the glass/Perspex and hence be flat.
One step further would be to add a hinge on one side and a catch on the other so it can be totally enclosed to keep dust etc out and make changing posters a bit easier.
You can get the moulding to make the trim quite easily. There are various shapes "Hockey Stick" being one. Cut to length and mitre the corners. Stick to the wall and then slide the Perspex in. If you use Perspex it will be lighter than glass so you might get away with glueing the frame to the wall rather than screwing.