Mystery Man on Film posted some advice for amateur writers. My favourites are:
Chances are that you won’t sell a screenplay until you’ve made a name for yourself by selling a screenplay.
Always use a standardized three-act structure or some variation of it that resembles the films you see in the theatres because you can’t get something produced if it’s not original.
Since only perfect specs are the ones that are purchased, make sure your write and rewrite your script several times until it is perfect - that ensures that your manuscript is ready to be rewritten once it is sold.