The number one tip I can give people is to understand the FastPass system. When we first hit a park, one of us runs off to the longest wait ride to get FastPasses for everyone. On a busy day you should always be holding FastPasses for something. As a family with young kids, even if you miss your FastPass window, the cast member is often willing to let you use them anyways if you ask nicely. As soon as we got to Animal Kingdom that morning, the family headed for Expedition Everest while I got Kilimanjaro Safaris FastPasses. In a little over an hour, we were able to ride both, despite not even arriving at the park until 30 minutes after it opened.
You need to understand which rides have long wait times and really plan your day if you want to ride those. There are even smartphone applications now that show you actual or projected wait times for different rides. Soarin' and Toy Story Mania run out of FastPasses very early on busy days. On a busy spring break day, we just weren't willing to spend the 60-90+ minutes in line to ride either of those again. If this was a rare Disney World trip for us, we'd have planned to attack one of those rides first thing when we arrived at the park.
Avoid the crowds. Look at the Disney World Crowd Calendar (from the authors of the excellent Unofficial Guide to Walt Disney World book) and see which parks are the most crowded on which days. On a really busy day, a big park like Epcot or Animal Kingdom will handle the crowds better than Magic Kingdom or especially Hollywood Studios. We did a couple of short-wait rides / shows in Hollywood Studios that day but it was so crowded that even walking around was hard with so many people. So we abandoned it and went over to Magic Kingdom. (Be aware that Magic Kingdom can and does become full enough that they close it and don't allow additional guests in.)
Don't follow the herd. Just shifting your meals so that you eat lunch earlier or later than most people can save you a lot of time and frustration. One of the things we've done with great success is take advantage of the parades, especially in Magic Kingdom. The ride wait times plummet during the parades and we're often able to walk right onto rides that still say 30+ minute wait times just by getting there right before a parade starts. You do need to be aware of the parade routes and not get trapped. Tomorrowland in Magic Kingdom is a great spot to be because the parade doesn't come through there.
Have a plan. Don't waste time wandering back and forth across the park on a whim. Don't miss things that you really meant to see. We like to have a rough plan of our "must see" items for the day and make sure to work those in if we can. If you really want to plan, get a book like
Don't go into "commando" mode. Don't be that parent dragging an exhausted, sunburned, and hungry kid across the park just to meet Ariel or ride the tea cups. Relax and enjoy your time. You can't do it all in a day. We've been to Disney World probably 50 times since our kids were born and there are still things we haven't done. It gives us something for the next time.
