1.Please please, please don't shoot the video in portrait. Rotate the phone to landscape. You will get the correct aspect ratio and as the phone is easier to hold (with 2 hands) in this position, a more stable image.
2. Try to get as close to as possible to the performer. The picture on these little lenses and sensors gets grainy really fast. Generally, folks in a club or coffeehouse won't mind. They know you want to get the budding artist on film "before they were famous."
3. Speaking of stability, it really helps to tuck your elbows into your chest and abdomen. Remember, someone may actually want to watch this after you post it. If it jiggles around and is unfocused most viewers will quickly move to the next of thousands of youtube videos out there!
4. Be aware of lighting and focus. Most smartphones will give you at least a bit of control. On the iPhone tap on the screen to lock autofocus and autoexposure before you begin shooting.
5. No sudden movements! If you pan, pan slowly, and twist your entire body with your elbows locked, tucked into your body. Otherwise your video will look like Les Mis, with a bad camera!
6. Audio can be a big problem. Try to be as close to the audio source as possible, whether it's the performer or one of the PA speakers, and don't chat while you're shooting. Your talking will spoil the audio of the performance. Also, these little phone mics don't hold up to loud sounds very well. If your friends are in the newest incarnation of Metallica you can try putting a piece of tape over the mic or simply covering it with your finger. It will color the audio but will help fight distortion.
7. If you're tempted to shoot at an arena show, don't! Unless you happen to capture Beyonce or Bieber taking a spill you will never get anything worth seeing and will manage to annoy the heck out of everyone around you. Take a couple pictures, put the phone in your pocket, and enjoy the show.
7. It really helps to roll the file into iMovie or Windows Movie Maker before you post it. You can edit and clean up the picture and audio, add fades, titles, etc., which makes for a better post. These programs are actually a lot of fun to use and can improve your videos greatly.
There is a lot of satisfaction in capturing live performance of our loved ones on their journey to the stars. Happy shooting!