In offering this advice, I fully acknowledge that I am both a reactionary and a contrarian. Since the beginning of the Hickman Revolution, in 1984, most RPGs have gravitated towards removing actual risk from the game in the interest of allowing the players to experience an ongoing narrative with some certainty that they will see its finish. Loss and death are considered "not fun" and excluded from the gaming experience. In Fourth Edition, Rust Monsters don't even destroy items any more. I believe that this, more than any other trend, explains why RPG combat is so often boring. If the players will always win, then why bother to pay attention or even care?
To be clear, I am not saying that every combat must put the entire party at risk of being wiped out. But I am saying that every combat should put the entire party at risk of losing something. There are three broad levels of risk:
When an encounter risks "assets," it means that survival of every player character is likely but the participants might lose things they value. This could be treasure, vehicles, pets, henchmen, magic items, and so on. An encounter with a Rust Monster in Classic Dungeons & Dragons is an asset risk encounter.
When an encounter risks "characters," it means that survival of the party as a whole is likely, but individual player characters might die or be irreparably harmed. An encounter with energy-draining undead in Classic Dungeons & Dragons could be a character risk encounter, as even a victorious party might experience real harm.
When an encounter risks "party," it means that the survival of the party as a whole is in question. Losing this fight will mean that every character is killed and the campaign is over.
If you've been "trusting in the Fudge", i.e. fudging your dice and outcomes, try putting something like the above at stake in your next combat. It doesn't take much to get the players to sit up straight and get involved in the combat. The moment they realize they could lose something, they start to pay attention. And if they realize they could lose everything, the increase in intensity is palpable. If nothing else, you'll be able to tell whether you've run a great fight by how the players react to someone getting the killing blow on the main bad guy. If they're bitter that they weren't the "cool kid" who got the kill, then there was no real risk - they were just competing narcissistically amongst themselves. If they are fist-bumping the player who got the killing blow and cheering with relief, then you ran a great fight.
The spectacle must be awesome!
RPGs are always described as games of the imagination, in which the players and the gamemaster weave the action and imagery with their words. And yet, all too often, combat is just run by the numbers: "Bob, roll to hit." "14! I hit it. I did 8 damage." "OK, the orc dies. Jim, you're up next." Such a fight can be intellectual stimulating, if you're a wargamer, and it can be worth paying attention to, if the stakes are real, but it is nevertheless lacking in emotional punch.