Planning - Planning is the most important when photographing Fireworks Displays and this is what most people tend to ignore. Always get to the location early. Be sure of where the fireworks are being setup and what part of the sky will it be lighting up. Try asking the organizers if possible of what they are planning. Look around the location and take note of the lightings and surroundings. You might want to decide now on what lenses and the focal lengths to use. Remember the first and most important thing in fireworks photography is planning and anticipation.
Framing and Focusing - Where to aim your camera? This is one of the most difficult part in photographing fireworks displays. You normally need to aim your camera before the fireworks goes off. I normally spend most of my time looking in the sky rather then looking at my view finder so that I could see whats happening around me and also anticipate the moment or the right time to shot.
Always manually focus your camera or put it on infinity. Its quite impossible to use auto focus mode in low light and you may end up missing a lot of shots. So set your focusing in advanced and fix your focal lengths but remember to adjust your focusing if you happen to change your focal lengths.
Shutter Speed - Its not necessary to set your shutter speed to a very low setting. The temptation to keep your shutter open too long is because its dark and you need to do that. The problem is that fireworks itself are bright and you might over expose them and you may end up not having a clean and nice shot. What you may get is too much of the smoke in you photograph.
Aperture - What aperture to use in Photographing Fireworks Displays? Many people thinks that they need to open up the aperture in order to capture them right. But remember, the lights that fireworks emit is quite bright. I normally set my aperture between mid to smaller f stop and i find it tend to work well. And again it will also depends on what shutter speed you have selected.
ISO - Shot using the lowest ISO possible.
Capturing the Mood and Surroundings - When photographing Fireworks Displays many people tend to just capture the Fireworks and forgetting the foreground or the background. Remember Fireworks Displays are often relates to celebrations and occasions and I personally feels your picture must relates to the occasions and it must tell you where, what and when. Don't forget to include other elements such as people, landmarks or other perspectives to make your photograph more meaningful and the Fireworks Display looking more spectacular.
Take as Many Shots as Possible and Track Results - Keep taking as many shots as possible and do a quick check occasionally. But remember..... don't check after every shot or you may end up missing the action!!! You might not have enough time. If you are using Digital Camera take advantage of the zero processing cost and keep on taking.