The next step is to figure out the best process for framing in a way that won't tarnish its value [...]
Impossible. Don't get me wrong, you can certainly
minimize damage to the poster by choosing a good archival framing method, but there is no way that you are going to have a poster in a frame for 25 years and expect for it not to sustain any degradation, however miniscule. Even with UV-protected glass and acid-free backing board, exposure to room lights can even cause fading over time. As BwanaDik mentioned, if you don't have a museum-quality climate control system, the environment can do all kinds of things with the paper. For example, if you have very humid summers and dry winters, you might find some very light rippling over the years. This is not to mention the effects gravity can have on a hanging poster over many years.
I say all of this only because, if condition is very important to you, I don't want you to be disappointed if, after several years behind a frame, you take it out and are devastated that the paper is in
slightly less perfect condition that when it went in, despite the frame shop promising you it's a "conservation frame." If you really want to perfectly preserve the condition of a framed poster, with zero degradation, that's impossible in my book. However, there are plenty of smart ways to
minimize damage to a framed poster. The "Framing & Storage" sub-forum is a great place to start. Lots of experienced people there can help.