First of all, before I get to the answer itself, I would suggest that you should talk with your players. Ask them about the game, what they like, and what not, but more than that, ask them why they like or dislike certain aspects of the game. Usually, they will tell you why they think that your battles are boring. But other times, they will suggest other areas where you can improve or areas that they want to see more. In any case, this is a great tool to improve your GMing skills anyway, so you have nothing to nothing to lose.
Another little thing- if your players tell you that they don't like battles at all, or if they don't get the kick anymore from those battles, don't let it destroy you. Just change the focus of the sessions. Battles are not everything that this medium has to offer.
Let's now go to the realms of the battles, shall we?
A great way to improve your battles is to find depictions of ones in films, TV or books and to analyze them. See what works for you in those battles, what makes you love them so much. For this example I'll go for one of my favorites, which is the first swordfight scene from Princess Bride.
First and foremost, in this fight scene we get a fuller characterization of 2 of our main characters, both Inigo Montoya and Westley. We get some of Montoya's backstory and some of his POV about the world. We also get a little bit of the same thing, backstory and POV, about Westley. Great battles do just that, they improve our understanding of the characters. They're not just there for being cool, but also to help the story in ways that only the battle can achieve.
"You are using Bonetti's defense against me, uh?"
A great way to make battles great is through the things fighters say while their fighting. It can be compliments to each other or maybe just throwing things to sting the other fighter, to make her lose her temper.
In the Princess Bride they compliment the techniques of each other. Through this the scene achieves 2 things: Firstly, they make the fighters seem far more professional, making them acknowledge the mastery of each other. Secondly, they make the fighters seem more badass when doing this, and this can only help in making the battle even cooler. But there's another benefit, again, which is characterization. When they say those things, we get a glimpse of their training, we get their fighting style which is a window to their inner selves, and we also get to see when they break the rules, which makes their cleverness show.
Try to make your NPCs and monsters say thing while attacking, try to make them nitpick or compliment or whatever the PCs. Maybe it will make the PCs see them differently, or maybe it will just make the PCs angry about the NPCs, thus making them more willing to fight.
"Cliffs of Insanity"
Places and terrain are other things that you should consider. But it is not enough to describe them, they should affect the gameplay. If you have a fight scene in an "Exploding-Dolls-Factory-tm" don't just describe it as being there and then leave it hanging, but instead use it. Maybe some dolls explode near the NPCs, or the PCs, or maybe the NPCs are using those dolls as weapons? If the fight is on a volcano, have the characters try to stand still even though the mountain is moving, make the breathing harder, or just have some bangs and booms at the end.
In the Princess Bride scene, they move from place to place through the terrain, trying not to fall, or jumping and using the things in there as defenses. It makes them look cooler, hell- it makes the entire battle cooler.
Try to reward your players for using the terrain, with little +1s here and there. More than that, set an example yourself- make your NPCs use the terrain also. It will make the PCs want to use it too.
"He flips over a beam and lands next to his sword"
Tumbling and moving is the key. Terrain is not enough if you don't move, and moving doesn't need a cool terrain to be cool. Have your fighters lip-flop from side to side, have them jump and surprise your PCs, and anything else you see right.
We see it in the scene too, in the action that I quoted from the script- He flips a beam and lands next to his sword. Do I really need to say more?
Have a note and stick it to your GM screen, a note saying: "make use of the skills". Set an example, and your players will soon follow. Rewarding cool moves could be extremely nice, have them develop a style of moving, of attacking and anything else. Even if the rules don't cover it. For these cases someone has invented the rule of cool- "if it is cool- it works".
"I am not left-handed"
Playing always right, fighting always as the rules suggest, is boring. Really boring. In the not-so-great movie Troy Achilles says something like this: "After you will master the rules, you will learn to leave them aside". That's a really great lesson; don't always make your villains play fair. Make them smarter than they seem, let them surprise you and your players. Not only the PCs, the players too.
In our scene, we see it twice- both of our swordsmen start the fight with their swords in their weaker hands. Why? Because they want to tease each other. They don't know that they both do it, that's the comic factor, but the important thing here is that they both try to cheat. Use it too.
"Please understand I hold you in the highest respect."
Last but not least, the end result shouldn't always be life or death. The battle can be for respect, or for benefits, or for money, or for acquiring allies. Don't always fight until one side is completely dead, decapitated and anything else you can think of. Maybe this time the enemies run away, or even better- they just surrender. Now you bring a moral dilemma to what seemed till now like a normal fight.
In our scene, Montoya asked Westley to kill him, surrendering and asking to be killed so his respect will be kept. Westley, on the other hand, lets him live. And suddenly we got a great recurring character, and again we got characterization. A win-win deal, if you ask me.