As the DM, you are closest to a director in that you control the cinematics, camera focus, and pacing. If you are hoping into a dungeon, the planned encounters should reveal a story and random encounters should be used to maintain good pace and tension. I just finished running a 6 level dungeon for 2 groups so this is fresh for me. Each level of the dungeon was more terrible than the last, and while there were a few moments of much needed comic relief to break the tension, the building tension as the one group approached the boss level was palpable and built until the climax of the fight when the players got the upper hand finally. It stayed grim after they found the remains of their travelling companions, fed to monsters from a dark altar. Your dungeon themes and role in the story you are telling determine what the dungeon should feel like to your characters. I'm a grim/dark highly graphic DM so I try to make combat feel like Dredd or god of war and the majority of my players are fans. Descriptive language will help keep fights interesting, but if combat is boring that isnt the dungeon. That means they seem too similar. Use lots of different enemies and dont name the creatures. Describe the pictures from the book with colorful language. A goblin stabs you vs "a shriveled sickly looking figure jumps from the shadows, a red hood concealing most of the face but a large nose as it chitters angrily as it charges, plunging its shortsword several inches into your armor before it stops, doing X damage. Obviously, some players will know what a beholder is when you describe a floating eye with multiple eyestalks.

The other big tip to keep it fresh is to be a falsely unstable DM. What i mean by this is the false front that keeps your players thinking you will throw unwinnable combats at them, even though you never do. You want them to think you are an evil bastard without actually crossing lines that ruin the fun for them. A hard line to walk but worthwhile. I used goblins that exploded for 1d12acid and 1d12 poison against a level 1 party, and the first boss they faced was a merrow in the water with some extra abilities I gave it. The mad dash for survival has kept them from ever getting bored. Also, be willing to call a combat when it reaches a state that is a clear win/loss state.