Never, ever, give anyone-friend, foe,and especially a professional-a script whose three digit page count has the middle numeral of "three." "One" is preferable. "Two" only if necessary. "Four" or higher means you should not quit your day job.
Who you should give your script to
When soliciting script comments from friends/colleagues, there are two categories. The first are those who identify the movie you're trying to write and give you notes to help you toward your goal, even if it's not "their kind of movie". These are good people to give your work to.
Who you shouldn't give your script to
The second category are those who read your script and give you notes about the movie THEY would write and/or prefer to see. These people should be taken off your distribution list because they will only screw you up.
Get written notes
Don't try to scribble notes down on the phone and don't try to retain them in your head. Get your friends to write them down. If they balk, remind them that 20 minutes at the word processor is better than a stressful two hour phone conversation. Once you get notes, put them aside and revisit them alongside other notes.
No repeat costumers
Once someone reads a draft, they're worthless to you because each subsequent draft will be commented on IN COMPARISON to the previous draft. Strategize who will give you first draft notes and then give the second draft to an entirely new round of people. Often just two or three people will give you more than enough to think about.
Be a pleasure to work with
Remember, the writer can be replaced, uninvited, and removed from the set, let alone the entire process. Have a brain and an opinion and a passion, but have a personality and a smile and a sense of humor, too.
Actors rule
I don't care who's watching your movie-a film professional or a cineaste or my mother- EVERYONE looks at the actors first. You can finesse that train sound in the distance on the soundtrack and be very proud of yourself for your use of irony, but if the audience doesn't like your actors, you're screwed. Cast well.
Eternal dilemmas
Scripts that read well on paper often make bad movies. Scripts that are difficult to read often make good movies. Development executives, agents, and script readers cast the worst actors in their head when they read your script. Actors cast the best actors.
College philiosophy that remains unchanged
When I made my student film I remember telling someone that I'd rather make a film that if 10 people watch it, three love it, three hate it, and four think it's ok, than have a movie that all 10 people think is ok. That's still my philosophy.