Figure out what gear you don't actually still need. If you think you'll want it later for cosmetics, or it's like "I have this 40 dungeon bow but my archer's still only level 10, although I will raise it later", stick it on a retainer; otherwise, slap an NPC in the face with it (either a vendor for gil, or your grand company's supply officer for free GC seals if it's dungeon drops) and get it out of your inventory!

It's really easy to keep around old gear thinking it'll be worth, but honestly it's also really easy to acquire common (ie non-treasure) gear again if you really want it; NPC vendors are not actually that expensive by the time you go back to pick up stuff for the class you are now using. So there's no point in keeping that white level 25 caster robe in your inventory if casters are on your "to do list" instead of "am currently doing", really