Rehearse the dialogue for the fight scene without the physical movements. This enables the actors to master their lines thoroughly before adding the physical elements.
Place the actors a safe distance from each other — only just within arm's reach for a fist fight, for example — and not close enough to hit their opponent's body with a sword if fighting with weapons, so the fake scuffle does not result in real injuries.
Mark the actor's placement on the stage with masking tape to help him to hit, and maintain, his mark.
Choreograph the movements of the fight scene in a similar manner that you would a dance scene. Both players have certain steps, arm movements and motions they will have to master in synchronicity. Work in action and reaction movements into the choreography. For example, if one actor swings his fist at the other, mimicking punching him in the eye, the actor being swung at must recoil at the right moment to make it look as though he were actually struck.
Watch the actors rehearse the choreography from the front and sides — at all angles the audience would see — to make sure the fight look believable from every view. If you only watch the fighting from one direction, you may discover too late that the right side of the audience can clearly see the punch feign from their viewpoint.
Rehearse the actors in the fight scene repeatedly. Practice makes the action and movement appear more natural. Work in any dialogue, including any grunts of pain or anger from the fight, as the players become more comfortable with the rehearsals.