Find a truly meaningful story. Why is this night --- or situation --- unlike any other? Aristotle believed that only the stories of kings and queens were worthy subjects of drama and that nothing else would interest an audience. We know that is not so. In dramatic form, we often tell the story of ordinary people but not ordinary circumstances. Search for a story that is truly meaningful, at least to the characters involved.
Show rather than tell. Exposition takes us out of the dramatic form. Instead of having a character say, "I am angry," show us, by having her walking away or yelling or quietly steaming. A strong visual image is much more effective than words. Some exposition may be necessary, but you don't want it to be obvious. If a man says to his wife, "My brother Jeremy, who is three years older than me and who liked you in high school, is coming for dinner," it rings false. She likely knows that Jeremy is his older brother and that he had a crush on her. It would be better dramatically just to mention that Jeremy's coming for dinner and find a way to show us more naturally that they are brothers, that Jeremy is older --- if that is important --- and that he liked her once. And maybe still does? It takes a little longer to do it this way, but it is much more satisfying and dramatic.
Write action. Action is plot. The actions a character takes in pursuit of a goal --- the choices and decisions made --- move the play forward and keep the audience engaged. Action is character. How a character acts informs the audience about who that character is. Action is drama. Everything in dramatic writing is tied to action. Make sure the actions of your characters are strong and clear and urgent.
Structure your piece with a dramatic arc. Whatever the length of your play or the number of acts within it, the dramatic form includes three parts: a beginning, middle and end. The beginning is an introduction of the characters and the situation. In the middle, the journey, struggle or search begins, with obstacles, setbacks, successes and defeats, culminating in a crisis. The ending is where a climax is reached, and the main dramatic question is answered one way or the other, leading to a resolution of the play, which often could be a new beginning. The action and arc of the play should rise and intensify, with just minor depressions, right up to the climax of the play, and then come down briefly as it ends.
Use stage directions sparingly. Unfortunately, it's common practice for many directors to tell actors to disregard all stage directions. You can't control that, but it is probably best to limit stage directions to the setting and whatever is absolutely essential, rather than following Samuel Beckett's example, where every move is prescribed. Shakespeare was a master at including almost all essentials within the dialogue, so he didn't need a direction that said a character takes out a knife. Instead, his dialogue said, "And with this knife I'll help it presently." Where there may be confusion about your intent for a line, whether it's meant in jest or with venom, include a direction --- or try to make the line stronger so there's no confusion. But it is best not to tell the actor how to express that venom. They like to find that for themselves.
Reread and rewrite. Read your work aloud to yourself and then have actors read it if possible. When a dramatic work is spoken, you can hear whether the dialogue is right or if it is overwritten or too formal. Overwriting is common in early drafts; it's how we find our stories. But now is the time to cut anything that's extraneous. Look for exposition and stage directions that can go as well. And be careful that you are not telling the audience what to feel. They like to find that for themselves.
Write in standard script format. The character name is usually in caps and is centered or indented. Stage directions are indented and in parentheses; avoid italics. Number your pages. Unless otherwise specified, 12-point type is usually requested. Break a leg!