My biggest struggle of coming back to DM'ing AD&D, OD&D and DCC were thus:
- BE the DM. Rule quickly, as fairly as possible, and don't sweat what the rulebook says.
- Stop rolling. Let the players figure things out without dice rolls as much as possible. Let the characters use their "knowledge" without rolling unless it's absolutely necessary. A cleric should have a good idea of the major deities and some of the minor ones. A wizard should have some knowledge of how the multiverse works. A fighter can figure out an opponent's disposition, strengths and weaknesses, etc. Thieves know value and risk...etc.
- Stop rolling, corollary: If someone takes the time to examine something and there's a secret something to find...why roll for it? The old school mitigated time spent with the possibility of wandering monsters, torch burn times, and the like. Figure out what works for you, but don't cheapen every room by reducing it to a series of rolls. Actions should result in descriptions....not necessarily a dice roll.
- Describe everything from the perspective of the characters. Remember that the characters have poor lighting, caves are complex formations of rock that don't reveal everything at a glance, some of the characters may be injured, tired, ability damaged, etc... Dungeons (and adventuring) should not be an easy process. All their equipment is heavy (regardless of STR). Conditions are harsh, etc... Make them feel it.
Many here would say roll in the open...I prefer a middle way. Some rolls should be private, in my opinion.
My modus operandi has evolved over the years to match Savage Worlds: Keep it Fast. Keep it Fun. Keep it Savage.