I break the action down into page by page descriptions. Like, "page 1: character A meets enemies. Swords are drawn. Page 2: character A swings sword, enemy ducks to avoid it." and so on and so forth. I do this so that I know what happens, when it happens, and how many pages I'm spreading the action scene out across.
Then I thumbnail it all out, to see how the action flows across the page - how to lay out the panels, how to keep the action clear and easily read, and to keep the reader's eyes moving across the page correctly. I do my very best to keep track of where my characters are in relation to one another, so that they don't turn to the left when their enemy is really on their right - and so on.
Here, for example: even if, in the bottom panel, the bandits are spread out across the panel, rather than on the right side, the main character is still turned towards the right, so that it is clear he hasn't turned around. If you don't keep track of where everyone is in relation to everyone else, it's easy to mess up and make it look as though people are teleporting from one place to the other without moving in between them, and it gets confusing.
Action-scenes still give me trouble, but I think I'm getting better each time I draw one. Practise makes perfect!