I pick up one of these for each deck. I like the two-sections-plus-two-dividers setup. I keep my current character deck (for OP) in one section, and behind the divider I put the boons up to the AD I'm currently playing. In the other section, I keep the characters I'm not using, and behind the divider, the boons above the AD I'm currently playing. (When I hit AD 6, I split the boons in half between the two back-halves of each section.)
I generally only carry one or two decks with me, so this scales fine. At GenCon time, I may think about something like the Caselings that Hawkmoon IronGiant mentioned.
I sleeve all my cards. It's become kind of a ritual when I get a new Character deck to go through it, reading and sleeving.