1. The most effective thing we do is avoid eating out. We always bring our own sandwich fixings and things, or stop at a grocery store to feed the family more cheaply on the road. This will bring down your bill a LOT!
2. Stay with family and friends. Over the years we've met a lot of people and they've moved on to different places in the world. We usually plan our vacations to visit them. And we love it when they come and visit us in return!
3. Go camping! So fun! And the kids love to play in the lakes and streams all day, so they're very easy to entertain!
4. Visit all the free sites! Do your research! There are always parks and museums and national parks that are really low cost or free. We always visit these first.
5. Bring treats to the sites so you're not tempted to buy them there, and also bring your own lunch.
6. If you must stay at a hotel, stay in one with free breakfast and a microwave/fridge, so you can cook meals there. Or some even have kitchenettes. It will save you more money in the long run.
7. Take the public transportation rather than a cab.
8. Use hotwire.com to get lower rates on hotels/travel/cars.
9. Travel off season to get better rates on hotels/condos etc...
Here are some more articles on traveling cheaply with a family:
10 ways to find cheap flights
Rick Steve's thrifty tips