Don't get suckered into selling your script without having proper representation. If someone offers to buy your script, get an experienced entertainment attorney, a literary agent, or a personal manager to handle the deal for you. The commission will be well worth it, and it will make you look like a professional. Don't just write one script and then give up if it doesn't sell immediately. Keep studying, writing, and promoting.
Timing is everything.
Don't become an annoyance to industry professionals. Persistence has to be tempered with judgement. Don't be demeaning to assistants. In this business, assistants become top agents and producers overnight. Treat them nice, and they will remember you when they become powerful enough to help you.