1. Shorter sentences, stronger words.

Keeping your sentences shorter and using stronger verbs/descriptors can help imply the urgency, power, and speed action scenes tend to have. There is a bit of a balancing act to be had to keep it from sounding choppy, but once you find the right variety of sentence structure and speed for your writing style, it is like magic.

2. There isn’t much time to think when things are going wrong.

Think about times when you tripped, or had a ball kicked/thrown to you during a game. Did you have time to take in anything but the event? Did you get more than a single sentence through your mind before the ground or the ball was right there? The same goes for action scenes, these are all happening very quickly and your character will not get much time to think about things before they happen.

3. Keep track of where the characters are.

And I mean this both in relation to each other and their environment. This can help you keep from getting things messy in your mind and help inspire more twists and turns.

For example, say you are writing a sword fight in a forest. The characters are likely to use the trees to help dodge blows and trip up their enemies, or they could be tripped up by a root and find themselves in a dangerous position. A third character could jump out from behind a tree and turn the tides of the battle drastically. If you are doing a car chase, for example, there are probably other drivers in a city street that could complicate things for your protagonist or your antagonist. On a deserted road in the middle of no where there may be less traffic but the roads are not as likely to be maintained, which could make driving at high speeds even more hazardous than it already is.

4. Look for other examples of the kind of action you’re trying to do.

Have a master thief leaping from building to building? Perhaps watching some parkour videos will help you. Sword fighting? HEMA videos and fantasy movies might be able to help you out. Car chases? Plenty of shows that feature the fantasy versions of them. Ect.

While watching videos of the kind of action you want to write, maybe try narrating it in your head, putting the visual into words. And it never hurts to see how other writers do these types of scenes as well. While reading, make note of techniques you like and ones you don’t care for. This will help you develop your own style.

5. Risk and reward.

Stakes can help get people more involved in your action. Is it life or death? Could the killer get away? Will the space pirates steal a priceless treasure if they board your characters’ ship? Could the guy who framed your protagonist succeed in his evil plots if they’re captured by the police before they can expose him?

The risks don’t always pay off, though, and that could be a fun thing to explore as well. What if the pirates do board? What if the protagonist is taken downtown before they can get the evidence they need? What if the knight loses the duel?

6. The greater implications.

This is not necessarily something you will be showing in your story, but it may be something you may want to think about. The greater implications are likely going to be a starting place for an action scene or the end of one, rather than the middle one. For example, your detective caught the bad guy in the end of your epic chase, but he’s not the one they were looking for. Implication: The real one is still out there doing their crimes. Your chase has been concluded, but your stakes have been risen, all the while you have set things up for even more twists.

And that’s about all I have on general action tips. I hope this helps you out, and if it doesn’t, I hope you find a method that works well for you =)