First, my tips for editors (from the point of view of someone who was a screening judge several times in the dim and distant past):
* The contest can draw 7-8 HOURS of AMV entries that must be sorted and filtered by a committee in a single day that can easily run over 12 hours long in high summer heat. It is not possible to watch every entry in its entirety, so first impressions matter a LOT. Hook your audience early (in the first few seconds) in order to have your whole AMV viewed.
* Read the contest rules and obey them. If in doubt, ask. With the judges under pressure to cut the raw entry bulk down to a contest quota, rules infractions become an easy way out of making hard decisions on the margins. Don't be that pretty-good AMV cut for pushing the PG-13 envelope, displaying a network logo in the corner, allowing fansubs to leak into your source video etc.
* As mentioned many times recently, brevity is the soul of wit. The less time you attempt to claim in the program, the easier it is to grant it. Be willing to edit your source audio, especially repetitive parts, especially if they're leading you to repetitive (redundant) video. If you hunt, you may even be able to find short mixes of songs you want to use.
* Your audience can't read your mind, so if the meaning of your audio and video is not blatantly obvious, then the AMV that moves you (the artist) to tears of joy may move judges or contest audience to yawns. There are several approaches to elevate meaning to clarity. You can choose audio with articulate wording (stand-up comedy, movie trailer etc.). You can add your own text (or symbols) to your video (there are artful ways to do that -- discuss amongst yourselves). You may think of yet another way, just don't rely on telepathy. Note: if you add words that look like they could be subtitles from source video, then include a note with your entry explaining that they are yours.
* Comedy: It seems like cheating, but using source audio that's already funny gives you a big head start. It may not win the category, but cleverly illustrating a polished routine can get your AMV into the contest (and entertain the crowd, our ultimate goal) with a lot less sweat than when you struggle to invent your own schtick and master the comedic timing yourself. YMMV.
* Music: While you can't please all of the people all of the time, choosing an "acquired taste" (e.g. death metal) may alienate some of your audience, starting with the judges. That's not to say that death metal can't get through (it does, much to my chagrin), but know that you're climbing up hill (especially if the words are not intelligible). The same, possibly double, goes for the opposite extreme (e.g. "elevator music"). You must be true to your inner muse of course, but you help your chances by aiming to be interesting without assaulting the ears.
* Warning against overused source material: Once upon a time, judges were told to allow only one AMV per category that primarily used any particular source. It meant that if two or more Naruto AMVs came into the intensity category, then only one could survive (there can be only one!). Same for audio (e.g. popular movie trailers). I don't know if that filter still exists (times change), but to improve your chances of getting in, you may want to avoid the usual suspects.
* On the other hand, the contest audience tends to lap up those hyper-popular sources, so you might deliberately pander to them. Just make sure that your AMV is the best so that somebody else's gets bumped.