Well, first you'll want something to capture your video.

http://www.fraps.com/download.htm

To make your videos look professional, you'll want to get rid of the HUD and the ever so annoying fact that names come up on the screen when you move your crosshair over them.
In Call of Duty, this can be removed with TWO (maybe ONE!) console commands. You need cheats enabled on the server so, it's best to have your own server or a friend let you borrow his server.
These command(s) are: cg_draw2d 0 (removes HUD and possible the names that show up, I think it does both, but just incase:) cg_drawCrosshairNames 0
When you want your HUD and Crosshair names back, just type these commands again, but with a "1" instead of "0."

For Counter Strike: Source:
sv_cheats 1
cl_drawhud 0
impulse 200
(Taken from the Over The Counter Strike forums. Thanks to tykell for these.)

These console commands remove all traces of the fact that you're filming inside a video game and sets apart a Machinima Newbie's movie and a Machinima Professional's movie.

Next, you'll want the server you film on to be PASSWORDED and especially with CS, you'll want round times set to unlimited and you should be allowed to be able to change maps at your will.
CoD should have map length time set to unlimited with cheats set to ON.

You also need a group of friends/clan members who are willing to follow orders and not pee-pee you off. Let me tell you, having people run around firing their Tommy guns randomly while you try to tell them where to go for the scene which leads me to:

Teamspeak... You and all your actors NEED teamspeak. Even if they don't have microphones they just need teamspeak. It makes co-ordinating and directing your movies so much easier.

Camera angles are important, do test shots but DON'T film. Just make sure you know you've got a good angle and if you must move the (high speed >.<) in game Spectator cams, then make sure it's not a complicated move.

NEVER film in first person unless it's a thing that requires running speed. You'd use CoD's cg_draw2d and cg_drawgun to remove the on screen items then use the camera like that, but that's the only instance.

Feature length is important, if it's a LONG comedy movie. Consider splitting it up into a series, red vs blue style, it also leaves viewers begging for the next episode if the current episode is entertaining.

Voice acting.. Yes.. Use teamspeak again and make sure your voice actors have GOOD QUALITY MICROPHONES. You can remove all that background noise with certain programmes, I've never had to use these programs but not everyone can afford studio quality mics.

Finally, movie editing. If you have Service Pack 2 for windows XP, you can edit movies for free and with good FX with Windows Movie Maker. It's easy to learn (a few hours and you'll be editing like a pro.) and easy to use. You have an easy timeline editor. I use "Sony Vegas" to edit my videos which is a great deal more complex. But it costs a hell of a lot of money and a lot of people don't have that amount of money to throw around.

Putting together a fun-to-watch movie.
Some annotations...

What makes a good movie are interesting characters and nice camera work.

When putting together a story, remember that there needs to be a beginning, middle and end. [Beginning] Introduce the setting and the characters. Pay special attention to the main character and provide the viewer with a reason to be emotionally attached even if means you just think the character is funny. [Middle] Here to present the conflict to the main character. The character is here for a purpose and this is where you want it stated. The middle also leads up to the climax, ever building to the high point of the movie. These scenes should make the viewer excited about what is coming next. Also, this is where the climax is reached the suspense ends. Here the conflict is resolved. [End] In the end you want to close up all the loose ends.

To help structure the story, start by writing down the main character's conflict in the movie. Think of why the character is here in this setting. Then think about if your character's motives and actions are relatable to a real person.

When going through the story, don't provide the viewer with more information than they need. This can just confuse them. What I mean is don't introduce new information that doesn't fit into what is at hand. Save it for another story.

When writing out the story:
1. write out the treatment that includes the characters and story
2. build the set and grab still frames for reference
3. print up the frames and lay them out in the order you want them in the movie. This is the best time to work out the story line and see what fits and what doesn't
4. capture the movie
5. edit the movie

Don't think about the camera as being tied to a single position. Through the magic of editing, you can tell everyone to freeze, movie the camera angle, then tell everyone to start up again. Another idea, if you have enough people helping you, is to have more than one spectator. Spectators can record their film and it's just like having more than one camera running at the same time. You may catch some exciting angles and moments during the film.

A Note on Camera Work:
When filming soldiers in a classic dialogue position, be sure to keep the eyeline generally one third from the top of the screen (One third of the way down your screen should be where the actor's eyes are located). Note however that in Long shots or action shots, this rule does not apply.

A Note on Recording/Filming:
When using Call of Duty for your film, it is INFINITELY more useful to DEMO your footage. Let me explain. When I shot my last video, I shot it in first person while recording a demo. This allowed me to maintain a high FPS while getting the shots I needed in a locked down server. I then used a program called seismovision to play back my demos in CoD2. At this point, I used FRAPs to record my footage into .avi. The advantage of this process is that you can get as many takes of a shot as you want without wasting too mcuh space. In addition, seismovision allows you to slow down or speed up your demo, so if there is a shot in ther very end of the demo that you want, you can simply fast forward to it in a matter of seconds. In my opinion, it is the greatest technique for filming your machinima.

A Note on Video Editors:
To be blunt: Windows Movie Maker is subpar. It is, plain and simple, a very very bad tool. I'm not saying its useless - it is quite possible to create a good movie using it. My point, however, is that it should be avoided.

High-End video editing programs offer much more flexibility and ease of use. The best video editing program on the market for consumers is Avid Xpress DV and Avid Xpress Pro. Both of these programs are industry standard, and are slightly stripped down versions of the Avid Symphonies and Avid Media Cutters used to offline edit major motion pictures. There are two drawbacks to Avid however. First, the program is extremely expensive. Second, the learning curve is relatively steep.

Coming up in number 2 for high-end video editors would be Apple's Final Cut Pro, which is slowly gaining on the lower Avid Xpress systems. I personally do not use FCP, because I work off a PC based system, but its reputation is solid.

Number 3 in the list of higher-end video editors is Adobe's Primiere Pro. Like all programs from Adobe, Primiere is a reliable and effective tool for video editing. It is widely used along side Avid Xpress and FCP systems for lower-budget TV productions (i.e., a News Show would edit together stories on either and Avid, FCP, or Primiere).

Coming in fourth on the high-end video editor market is Sony's Vegas. It is a sophisticated system in comparison to consumer brand programs such as Pinnacle or Windows Movie Maker, but it has yet to catch up to the flexibility, reputation, and global recognition of Avid, FCP, and Primiere. It does, however, hold storng promise for the future.

Quote on Editing Software, get Camtasia Studio, its great for editing.