Cypress and coco coir are my favorites for retaining humidity. Cypress comes in larger chunks while coco coir has a soil like consistency. I like to do an inch or so around most of the enclosure, but you'll want it thinner over the hot spot so your snake can reach the heat. Both of the aforementioned substrates start out moist, but if they dry out you can always give them a gentle spray witha spray bottle to increase humidity.

Poop is usually pretty easy to find and so are urates, for liquid pee just look for a dark wet spot in the substrate. Replace everything that is soiled as soon as you see it. How often you do a complete cleaning will depend on several factors, like how often your snake relieves itself and how well you spot clean. When it startes to smell no-so-fresh it's time to do a full cleaning.

I usually dump out substrate and then wipe the cage down with disinfecting wipes. Nothing fancy, but it works for me.