If you’re aiming for a more professionally produced machinima piece then the following tips are for you, in no particular order…
Hardware
Use a beefy desktop if possible. You need a decent graphics card and plenty of RAM. A fast hard disk really helps capture smooth video, slow disks and low RAM will cause you headaches as your buffers will fill up before the video can get writen to disk.
Record in as high quality as your machine can manage
If you’re filming on a decent desktop machine and it can cope then go for 1024p resolution, this will give you the best quality playback if you’re projecting onto a large cinema screen or using a High Definition plasma/LCD display. Go for 720p as the next best option (this is what i use most of the time)
Playback at an Event (trade show/conference)
Always playback from a computer hooked up to the monitor/projector. dont be tempted to put the machinima on tape or DVD the resolution is much worse than you’d think. If doing a live broadcast event using the video have a machine for backup, as last resort use a tape or DVD backup.
Hiding the UI
Unless you’re doing documentary or event capture where you need avatars chat text you’ll want to hide the UI, do this using CTRL-ALT-1
Avatar chat
If you’re filming a scripted piece then put the captions on afterwards, the standard SL chat is small and hard to read when playing back at a lower resolution save your viewers eye strain and add the captions afterwards along with any other titling. This also gives you the option to quickly do changes when your client comes back to you and requests a script change (which they will).
Things to tell your client
Make sure they know that filming machinima is a vary similar process to RL filming, you’ll need props, actors and a script. You’ll have to do multiple takes and post editing. It all takes time. If you are able to have influence on the script etc try and keep the number of characters in each scene to a minimum.
Actors
As mentioned before, if you can keep actor numbers low do so, if you’re lucky enough to have access to multiple machines use alts and control the characters yourself when numbers permit. I use three machines on occasion which lets me use 3 avatars with one being the camera.
If you do end up using a lot of other avatars try and get them to all go on a conference call. its much easier directing by voice rather than typing when you’re trying to use you avatar as a camera.
SecondLife tools
Filming Path HUD is essential for getting smooth camera shots – panning, top shots etc. it’s the best tool around and allows you to lay out the path you want your camera to follow. You can set a focus target on another avatar to smoothly follow them while they and the camera moves. You can also set static focus points so that you your camera will stay fixed looking at a set point even when moving through complex curved paths etc.
Be creative with angles (but don’t over do it 
Filming machinima allows you the freedom to use creative angles more easily than RL filming. try shots from above, below, panning, swooping and aerial. make sure they will fit together nicely when you edit afterwards though i.e. keep continuity in mind.
Don’t forget composition
If you’re new to filming or photography then look up some of the composition rules of thumb, leading lines, rule of thirds etc.. once you know these simple guides you’ll spot them turning up again and again in hollywood films. They’re not hard fast rules but they work as a starting point and will help make your machinima a more visually pleasing experience for the viewer.
Film more than you need
play with different angles and capture scenes more than once. You can edit afterwards and pick the best pieces to use, if you have a script change then some of the excess video you’ve captured might come in useful.
Hope some of these come in handy in some way, if you have any more tips then please add them in the comments 