Hopefully, you will never find yourself in a situation where you need to live on a deserted island, but it sure beats the alternative: dying on a deserted island.

[edit] Steps

Calm down. The most important thing is to remain calm and think clearly. If you get to panicking, you can lose control and end up worse than you started out.
Evaluate your resources. Do you have a source of fresh water or a desalinate? Do you have a long range radio, satellite phone, or other means of communication? Are there other people? People, if managed correctly, can be your greatest resource.
Appoint a leader. The leader's job is not to come up with ideas, but to implement the ideas of the group. This is important for long-term survival. Mabye change your leader every now and then.
Start a fire. This may seem like a trivial thing on a deserted island, but fire has a number of uses. At the very least, it is a morale booster. You have accomplished your first task and that is key for morale. Fire can also be used to distill water (more on that later), cook food, and provide light to you as well as to rescuers. If you can't start a fire, don't worry, just move on to the next task and keep trying.
Build a shelter. This gives you a home base to return to and somewhere warm/shady to sleep. It also give you a place to store any resources you may acquire.
Look for water. Any source of water is good. As long as you have water, you can purify/desalinate it.
- If you have access to fresh water, just boil it for 2-3 minutes to be sure
- If you have a desalinate, congratulations. If not, there are several methods to desalinate it.
  - One method of desalination is distillation. You can construct a solar still, or one that works over a fire.
  - A solar still can be constructed by filling a large, flat container with salt water (or urine..It can be re-usable if need be).
    - Place a smaller container in the center with a rock in it to hold it still. #*** Cover it with a thin sheet of plastic or whatever you have and place a rock in the center above the cup.
    - If this is placed in direct sunlight, water should evaporate and condense on the plastic running down and dripping into the smaller container.
  - For one that works over fire, all you need to do is create steam and condense that steam by using a piece of metal or glass in the steam that allows the condensed water to run into another container.
Arrange rocks or large objects on a clear path to signal for help. The recognized mountain distress signals are based on groups of three (six in the UK). A distress signal can be 3 fires or piles of rocks in a triangle, three blasts on a whistle, or three flashes of a light, in succession followed by a one minute pause and repeated until a response is received. Three blasts or flashes is the appropriate response. If you are clear view of boats attempt to make a large red X.
Find food. Food is the lowest priority. The human body can last for weeks without food. There are lots of edible roots and berries, but watch out! Some are poisonous. Only eat them if you are sure that they are safe. The best, and most reliable source of food is bugs. Yes, bugs. They are everywhere and are an excellent source of protein. If you decide to go fishing with the bugs instead of eating them, you need a hook. One can be fashioned simply by carving out a stick into a hook shape and putting a barb on it. Tie a piece of string to it and you're in business.
Try to make contact. Build big, unnatural looking shapes using bright colors and shiny things. If you have a radio, use that to contact possible rescuers. Use signal mirrors and fires and flashlights and whatever else you can think of to draw attention to yourself.
Never give up. Desperate acts can be lethal to you and others with you. Willpower is what allows people to survive without food for weeks. If you don't have the will to live, you won't.

[edit] Tips

Each individual situation is unique and needs its own situational analysis. Take time to think over what you have, who is with you, what you need, and things of that nature.

[edit] Warnings

DO NOT drink salt water. The salt dehydrates you and makes it worse. If there is no fresh water, drink your own urine. It may be gross, but it will save you.

[edit] Things You'll Need

Knife! A knife is the most versatile tool ever invented. There is nothing you can't do with a knife. If you don't have one with you, make one. Breaking rock is a good place to start. If you are really keen, and have lots of time, and just so happen to be on a volcanic island (lots of iron), you can make a steel knife. But thats a whole another story.