Tyre pressure. Coming from a roadie/tri background you'll be kind of ingrained to pump them way up. Fine for road, very much not fine for dirt; you'll be bouncing & slithering all over the country side. Drop the pressure as low as you can while still maintaining control. If you go too low the tyres will get squirmy and handle like a bowl of jelly, plus you'll be more prone to pinch flats, particularly on lumpy stuff. Start at the low end of the tyre manufacturer's recommended pressure range and experiment from there. Set your rear a few PSI harder than the front, firstly because the back of the bike is more heavily weighted, so needs to be compensated for; secondly, the back wheel is more likely to be bashed around harder - you lift the front over a bump & slam it with the rear instead - so you need a bit more air for bump protection; and thirdly, coming back to the original point about grip, slightly higher rear pressure can make the back end just a little bit more playful, which helps keep the front planted & pointing where you want it to go, and a rear drift is easier to control than a front drift.
On hills, keep your weight toward the high end of the bike; bend your elbows & pull yourself forward when climbing to keep the front wheel planted so you can steer; when descending push yourself to the back of the bike to keep the rear wheel on the ground, so you don't descend on your face.