Familiy holidays should be about doing things together. Strike a balance between obviously 'kid' centred activities and stuff for the grown ups. Too much in either direction will mean that some members of the family will not be having a great time.
A first step is to search out cheap airline tickets. Why spend a fortune on the journey if it leaves you skint when you land?
Kids get bored. Plan ahead and bring some games, books, pencils etc to distract them on long car or air journies.
This is the big tip for adults and teenagers. Agree an 'everyone joins in' rule before you travel. Make sure everyone joins in, no matter how uncool an activity may appear at first sight. Remember holidays are as much about the memories as they are about the time itself. It would be such a pity if a member of the family was missing from a surprise activity that turned out to be fondly remembered.
Holidays are not just about going away from home. They are about going to new places. Explore your surroundings. Take the kids into local towns. Walks in the countryside allow them to see something very different from their familiar home. You might just learn something too.
Not everyone speaks English. Make sure all the family learn a few words and give points and prizes to the kids for attempting to communicate in languages other than English. It is great fun and really helps them, at school too. At the very least learn how to say hello in the local language.