I've making videos (a more fitting word than "movies" which sounds very professional, and definitely better than "films" which implies great creativity :) for about 10 years. I still make noob mistakes but I have acquired some wisdom from my mistakes. It's here for you to read but mainly for me to remember.
Disclaimer: people who are actually professionals will not learn anything here. This is for perma-noobs like me.
This one is obvious for most people who've made videos before, but I still see people's work produced with the camera's cheap mic---which is horrible btw.
Also, make sure you get a fuzzy for your mic. It will help reducing wind noise.
To check the sound levels and to make sure you're not picking up extra sounds (the air conditioner outside the window, or the plane going by, or the person walking down the hall) when shooting---especially an interview. Get some headphones that cover your ears (not earbuds).
When you shoot, take everything with you, no matter how late you are and no matter how long it takes to get everything into the car! Take your tripod, even if you're sure you'll only do handheld. Take your charger, even if you're sure you'll be outside and your battery is charged. Take extra tapes. Take extra batteries. Take headphones.
Take extra batteries everywhere! Make sure you get good batteries, like ones from your own camera company. Also, make sure you carry around batteries for your external mic.
When you're shooting an interview, it's probably best to use the auto-focus to set up the shot, then switch to manual so the auto-focus doesn't throw you for a loop.
Also, when shooting an interview, check to see if your camera has a portrait mode, which probably reduces the depth of field for that "soft background" look. Manual focus is especially key here.
If you have two interviews, you want the sound of the people's voices to be about the same volume in both clips, but unless you shot them in the same environment on the same day, this is not going to happen right off the tape. Adjust the volume of each clip until what you want to sound like normal volume is at -12db.
At the front of your video, include the color bars and tone (which happens to be at -12db, see above) so whoever's showing your video can easily calibrate for color and sound. Really you only need this if you're showing your video at some professionally produced event (like a concert or something).
No not the Mos Def album :) When you're showing the video at a professionally staffed event (like a concert), give the video/sound guys a break by including some black and silence on both ends of your video (so, after the bars on the front :) This will give them some time to show/unshow your video on the screen.
Music can make mediocre videos great. Do not include commercial music without a license. Go to places like stockmusic.net ($30 a track, pretty good quality) and gettyimages.com (more expensive but better stuff), to get stock music with an appropriate license.
If your video is to be shown at a concert, their professional audio equipment can reproduce sounds your editing setup cannot---namely very low bass. You may want to add a highpass filter at about 80Hz to take out very low bass sounds.
Before you get to the last week of editing/production and finally showing (some of which inevitably happen on the same day :), know your entire workflow---from filming, to getting it off the tape, to basic editing skills, to getting it back on the tape/into digital format after you're done.
In 2008, for the BACC Expo, I didn't know how to get my HD video out of Premiere and back onto tape---I had to render to some digital format and play off the computer, which was lower resolution :(
That brings me to another point: know what digital formats to export to and which ones you can produce fast. This will help you at crunch time if you need a digital copy.