I use my saddlebags for mats I need in the immediate future, materia (for either mutamix or melding), items I'll want to use (like maps, dyes, glamour prisms, fishing bait) but don't want to have to search the retainer for. For retainers, I keep anything that I know I craft with regularly that cannot be easily gathered (timer, folklore, bought with scrips/seals, only brought by retainers, etc.) If it's something like a furnishing you won't be using, doesn't fit in your storage, and you can make again relatively easy, vendor (or mb, if it's not bound). I highly recommend using the glamour dresser and armoire as storage, but as people have differing views on that, I'll just say that it's helped me immeasurably.

I do keep a lot of GSM materials (I'm the main GSM for our group, and I use the stuff a lot) and CUL, because oh my god you need so many, having to gather 24 things just to make a round of foodstuffs makes me want to quit the game. That's just personal, though.