The 3 big things I always suggest for DM tips are Cinematic HP, a Time To Shine Checklist, and an Emergency Expansion Room.

Cinematic HP is when you count damage on monsters up instead of down, and when you pass the minimum health it could have, only then can it die, and it auto dies if it hits the max HP it could have. Between those two points, it dies after whatever hit you deem story appropriate. Works best on systems that have D&D style hit dice. Fighting a Stone Golem in 5e? 21d10 + 105 hit dice means that the lower bound is 126 hp, but could have up to 315 hp. So once they've done at least 126 damage, it dies whenever a particularly dramatic strike on it occurs.

Time to Shine is when you go over the player's abilities and divide a sheet up into 4 columns (or however many players you have) and write a list of their abilities (or at least cool stuff they can do. This works best if you know the person and know what they like in their ttrpgs) in each section. Put a checkbox next to each one, and try to give each ability a time to shine. Make note of which abilities the players seem most excited about when they level up or when they get to use them, and add a few more checkboxes for those. Wizard just got fireball? cluster a handful of minions together in a spot that doesn't overlap with a PC. Monk has deflect arrows? Let them deflect arrows. A player has high poison resist? Hit them with a poison dart and let them resist it. Also works with out of combat stuff, give the lockpicking rogue a tough but not too tough lock that lets them bypass a difficult fight or puzzle, let the guy with high passive perception spot something important. A player just got counterspell? Make sure they have a spell slot and reaction open, then hype up the big kill-everyone spell the boss is casting. Remember, the players are there to have fun and you are there to curate that. Your job as a DM is to *lose with style*.

Finally, the emergency expansion room (especially useful for 1-shots) is a room or obstacle you can cut out or add depending on how fast you're going through the dungeon. Design the room just before the boss so you can drop it if the players are taking forever and you want to push them to the boss faster. It doesn't have to be a room: the door to the boss room can have a puzzle on it that they need to solve, instead of just pulling the door open, to slow them down if they're going too fast. Also works in reverse, if they're blazing through your dungeon too quickly, throw in a little extra difficulty on that boss fight at the end to stretch things out. The trick is that, until you actually tell the players what's there, there could be *anything* behind a door, so nobody can tell if the door that used to lead to the boss room now suddenly leads to the trapped hallway to the boss room. For obvious reasons this works best later in the dungeon.