The first question I have is what do you plan on doing with the poster?
If you need the poster to remain in a state so that it can be rolled up again, then I would suggest purchasing a piece of foam core or matboard from Hobby Lobby [Michaels, etc.] that is slightly larger than your poster. You would then unroll your poster, place the matboard on top and then add a bit of weight to the matboard. This will act as a press for the poster. It'll probably take a couple of days to retrain the paper to lay flat [slightly less with humidity and heat].
If you are going to hang the poster on the wall and leave it there for an extended period, I would suggest going to your local frame shop and have them mount the poster on the foam core or matboard. This will certainly flatten it out and provided it with some rigidity. They will use a spray adhesive and vacuum mount it to the board so, you won't be able to remove it or re-roll it.
My wife and I own a frame shop and that's how we do it anyways.