Reduce temperature changes, so you aren't as prone to moisture formation via condensation. Reduce time - you don't want a lot in storage for long, because it increase the opportunity for things to go badly. If you do get mold, BLEACH throughly before putting fresh feed in - otherwise, you are merely introducing fresh feed to a lomd positive environment.
(FWIW, I'm in sunny FL. Highs routinely in the low 90s, we are in excess of 10" of rain in the last 30 days, and mid 60s% humidity is our daily low)
For small quantities of feed you can use a plastic bucket with a lid. Larger quantities, many find success with a dead chest freezer - relatively inexpensive, rat proof, and if the seal is any good, it will protect it from significant changes in temp and humidity. Igloo plastic coolers also work, but expensive.
I keep anywhere from 150-600# of feed on hand at a time, right now I try and keep it under 350# - which is less than three weeks worth. I had to build a metal closet to rat and mouse proof, can't keep the roaches out - but when I do m every three-4 week clean out, inventory, and reload (FIFO), I let my chickens in to eat up any spilled feed and every crawly thing they can find. Win/WIn.
Good luck in your efforts!
