Plan everything as specifically as you possibly can, especially when starting out as a filmmaker. Plan your budget carefully, and stick to it as closely as possible. Plan your shoot schedule and do your best to stay on time. Even plan your shots using some basic story-boarding. If inspiration strikes on the day of filming, you can vary from your plan, but a solid storyboard ensures you have a good place to start.
When it comes to camera movement, less is almost always more. A common beginner trap is to try complex moves with a camera or to reach for a stylistic camera movement that just ends up coming off as illogical and unmotivated. Keep moves to a minimum, and only use one when such a movement truly serves the story. Concentrate on telling the story you want to tell rather than imposing stylistic choices and you will end up with a better product.
Never forget that sound is a very important part of the overall experience of your film. Filmmakers who don't pay enough attention to sound during the initial shoot either end up spending huge amounts of money on ADR sessions, or are simply forced to put out a film with sub-par audio that sounds unprofessional. A dedicated sound man with high quality equipment can take an amateur production to the next level, and is essential on a professional shoot. This applies to films where you're recording audio to a separate track and when recording sound to camera.
The one thing that can derail your production faster than any other is a legal conflict. This is especially painful if it occurs after you've done all the work. You must get legal clearances for everything that you use in your film, and have them all documented. This includes actors, music, locations and any logos or trademarked images that appear in the film.