At the start of combat, roll a d4. In that many rounds, something (usually something bad) will happen.

I once had a GM whose house rules included every battle receiving enemy reinforcements each d4 rounds. One combat took three weeks to resolve. At the end there were four factions in the ruckus and it was taking about 60 minutes per round for him to run it. After the first evening we weren't even trying to fight anyone, we were just withdrawing.

So a lot of these suggestions need "...but not to the point of bogging down the game" added.

When you make a ruling, explain your justification out loud. If you're targeting a random person, tell the players the stakes before you roll. "OK, evens the wyrm goes after Faust but on odds it'll go for Drena even though she's on the ground."

Saying what you are doing at all times is the absolute best thing you can do in my opinion, because it makes it clear to the players what your intents are and promotes honesty.

I would spice it up with an in game description, such as the dragon flicking his eyes between the two and saying "The wyrm flicks his eyes between the two characters. Evens for Faust and odds for Drena (rolls). The wyrm dives to the ground at Drena."

In my combats I will have monsters convey reasons for what they do, such as a melee character saying "A challenge!" when focusing on the fighter over the bard or "Out of the way!" if they take a swing at the Bard before focusing on the fighter. Goblins screaming in fear before disengaging if they survive a hit, or even a humanoid yelling "Fall back!" to make it clear why they are making a coordinated retreat and allow the players to end combat some other way than just killing everything they come across.

Scatter ideas… players are unlikely to move in any specific order or direction because that’s what’s expected. Most experienced players are terrified to do what the GM wants because they believe they’ll all die. Players and GMs are commonly perceived as adversaries, but they should be collaborators. So the GM can scatter ideas and little bits of business al over the place and then the group can choose. Members of the group can ask for elements and the GM can choose. Create the series together, allow the players to affect change in the setting, and never let an NPC do what a player does. Thanks for coming to my TED Talk.