As part of a Machinima clan, I can help a little. We had some success in videos.
If you want to get rid of the full hud, you need to find someone with an AR-code for Hudless. It's easier and you wont have to letter box the stuff you shoot.
Either way you get rid of the gun, beware of weapons on the ground. You walk over one and you will pick it up. Have an extra person designated the "Weapon Wrangler"
Beware of things like, "Gained the Lead!", "Play Ball!" or "New Juggernaut" popping up on the screen. If you want, let one person get a huge lead 12-0. But that's always fun, sometimes people don't want to be losing for any reason, even if it's machinima.
Make sure your cast and crew want to help, and not jack around - lots of people lose interest very quickly.
Make sure you realize that it can take about 2 hours to get 10 minutes of footage that is good enough to use.
Don't use emblems, unless the emblem is identified with a characters name or squad. Random emblems on your characters look out of place and odd.
Get audio in the pre/post-game lobby. It's easier and sounds cleaner.
You can't adjust lighting in the Halo world - so you have to use the light that Bungie gave you. Don't put a Steel Spartan in a shadow on Backwash unless you really mean to do it.
If everyone you are filming is a Spartan, you be an Elite and vice versa.
Don't put up with any s**t. If people don't do as you ask/instruct, boot them. Someone in your party is bound to have 2 controllers, they can do double duty.
Beware of weapons/grenades/vehicles randomly spawning in the middle of a scene. Put the "Weapon Wrangler" to use. (ie - Make him be on the other side of the map playing with the vehicles so they don't respawn where you are.)
They aren't great - because we ran into a lot of the problems that I discussed above (people being asshats and not helping) then I just quit caring. But I am attempting to put together a quick Halo 3 video (if i get off my butt....)
You be the camera man. Only use someone else taking the footage as a last resort. What you tell them to capture and what they capture could be the difference between night and day.
Best time to use an extra camera man is in a scene with a lot of people in it - have the extra camera man somewhere randomly in the group getting a shot from his POV. Then on the reverse, he wont be missed.