Anabelle,
If you're staying on property and can get up early, go to the early entry park (for that day) 70 to 90 minutes before the official opening time (for the general public). That way, you can experience popular attractions before the lines get long. If the park gets crowded later in the day, switch to another park with your hopper passes. If you want to spend a lot of time at the water parks, get to one 30 minutes before the opening time one day (or two days); you'll get to do a lot of the slides before the crowds get thick. Otherwise, go to a water park in the late afternoon or early evening; the crowds often thin a bit at these times. If all the parks are crowded, concentrate on shows (get seats 30 minutes or more before showtime) and use FASTPASS as much as you can. Eat early for lunch and dinner to avoid the lines, or have priority seating times scheduled before you arrive. I have fun at WDW no matter how crowded it is. You just have to scale back your schedule some and don't wait in any long lines if you can help it (I don't like to wait more than 20 minutes in any attraction line). Have Fun!
Steven Barrett, author of "The Hassle-Free Walt Disney World Vacation" and "Hidden Mickeys, A Field Guide to Walt Disney World's Best Kept Secrets", both books published by The Intrepid Traveler Publishers.