If leather gets too wet: Dry it slowly. Speed drying leather changes its chemical structure, and you end up with stiff crinkle cut chaos. So room temperature with gentle air works better than direct heater time with hair-dryer. And keep it in the shape you want it to end up.
If leather gets too dry: Rub something moist into it. Pick a glitter leather dressing or cream, preferably recommended by the maker. Leathers can have paints, waxes, oils and all sorts of things applied to their surface, so you probably want to pick something similar to how it came. Personally, we mostly use Dubbin for heavier use applications (because our dads did), or a lighter leather cream for wallets.
If leather gets dirty: Just use a damp cloth. You don’t want to be putting any soaps or foreign substances in there.
Leather stretches out, but not back: If you start to overfill a wallet, it will never really return to it’s former taut self. If you stretch a leather bag when wet or very humid, it will move even faster. Just learn to carry the right amount, and this takes care of itself.
Don’t wrap it in plastic: Going back to that Cryovac analogy, it can help to think of leather as having the life frozen rather than completely killed. Hans Solo still needed a tiny bit of air, and so does your leather. What you’re really trying to stop is any mildew growth, so keep some ventilation going.