My main character is lv50 in everything but BOT, FSH and MNK...and will soon be lv50 in everything but FSH (don't feel like leveling it)

I have 2 retainers. That's it. 2.

And I have free space in my inventory and on both retainers. Not much, but some.

My one retainer has gear and housing items; most gear I can honestly get rid of, and did get rid of a bunch of things yesterday. Some things I'm saving for my MNK, and will be turned-in after I actually go back and farm Coil again for what I'm missing.
My other retainer is full of crafting mats and materia; again, tons of stuff I will never use or need to convert (hides, logs, stone, etc), I've got a bunch of stuff vendors sell (no idea why), and can go transmute a bunch of my crap materia.

Its all inventory management. I don't (well, try not to) keep mats that just aren't used, and keep supplies of the "common" mats (Iron Ore, Mythril Ingots, Dew Thread, etc) around.