Outline your story ahead of time. Work loosely from your outline. A screenplay typically runs one minute per page, with a two-hour movie coming in at about 120 pages. The time per page can vary slightly, depending upon how a director shoots, but this is industry standard timing and why screenplays are strictly formatted. A three-act structure will consist of a set-up culminating in an inciting incident (30 pages), 60 pages of the protagonist pursuing his goal, hitting a road block, and rising action, which culminates in protagonist making a comeback. The last 30 pages consist of the protagonist pursuing and achieving his objective. These are guidelines only. Don't focus so strictly on structure that you let your story suffer.
Keep your story linear when possible. Flashbacks typically require a screen title or a some other indication a flashback is happening. Flashbacks interrupt the flow of the story and can confuse the audience or cause them to lose interest. Flashbacks happen in movies, but use them only when there is no other way to tell the story.
Write dialogue that is realistic and convincing. Pay strict attention to making each character's dialogue tailored to fit the character. Sometimes it's helpful to write dialogue with a specific actor or actress in mind. The downside to this is you could focus so much on an actor or actresses's style that you lose sight of the characteristics of the character you're writing dialogue for. Find the balance that works for you. Primarily, keep the dialogue natural and try to keep it simple. Overwriting dialogue can cause it to sound stilted and doesn't leave room for an actor. Avoid using dialogue simply to convey information to your audience. Show them the information with action when possible.
Avoid parentheticals in dialogue unless absolutely necessary. A parenthetical is a parenthesis used primarily for a short action that happens as a character speaks. Beginning screenwriters often use this to direct an actor on how to speak the line. This should be avoided.
Avoid writing camera directions and transitions unless they are absolutely necessary to the scene. Directors will likely disregard your camera directions. Scene transitions, which are right justified and include direction such as FADE TO and DISSOLVE are not necessary. These are decisions typically made during the filmmaking process.