Let me start by saying that the #1 destroyer of Pulps -- or any paper items for that matter -- is sunlight.
Keeping your Pulps away from UV rays is the most important thing you can do to preserve them.
Beyond that, here's what I use to store my pulps:
Bags are mostly protection from handling and other abrasive forces; If your
collection is on open shelves, they also protect from dust and other environmental agents.
(Some people don't use bags at all if their collection is in boxes or cases.)
If you are using bags, they must be made of an inert plastic like Mylar type D or Melinex 516. It's better to use
no bag at all than a bag made of polyurethane (the most common kind), since this plastic can break
down over time and react to the materials it's supposedly protecting. The size you're looking
for is the "Golden Age" (comic) size: 7-3/4"x10-1/2".
I buy 2mil. bags (Mylites2) from E. Gerber.
It's been suggested that the bag not be sealed, so the Pulp can "breathe" with it's environment,
not trapping moisture or stewing in a potentially acidic container.
Backing boards help keep the Pulp flat, and from resting on it's bottom edge when displayed upright.
If you didn't think they could do better than "Acid-Free" -- well they have. Now you can get boards with
a coating that actually neutralizes the acid in the paper it is near! I buy Life-X-Tenders (7-3/8"x10-3/8")
from Bill Cole Enterprises.
I saw this promoted for comic books and thought it would be even more valuable for Pulps:
A thin, acid free sheet of paper to protect the cover from the more acidic interior pages. I bought
some from Eclipse Paper and cut them down for each Pulp
(since they vary), but Eclipse will do custom sizes.