Shake up character setups. Give your characters novel surroundings and interactions. Writing yourself into common, overused character situations will lead you toward writing clichés. Be deliberate about your plot and setting, and you'll head off the cliché coming around the curve.
Identify sets of natural moments. When you are writing a scene, think about how mainstream writers would write it. Then think about the way it would happen in real life. Come up with your own mix and strike a balance to create original character dialog for your scenes.
Deal creatively with dialect, profanity and other dialog color. You may write characters you would describe as coarse or uneducated.
Say it a different way. When you spot a line you think sounds too common, try a quick fix: change a few words, but not the concept, and see if the dialog is improved. Always check to make sure you're not writing wooden dialog that a character wouldn't say, as sometimes, reactionary dialog lines can be pretty random.
Come up with inventive, original screenplay resolution recipes. The endgame of a screenplay is very important as it sets the final mood but be sure to insert your own signature. Serious writers create twists and turns that show off their conceptual writing skills. Lean toward the latter and avoid the feel-good, predictable ending.