I get asked why my photos aren't packed full of people. The answer is easy: I just ask people to step out of the photo as I'm taking it, and my wife assists by holding up yellow Police tape to cordon off the area.
Seriously, it's quiet easy. You just have to get to a park right when it's opening or right when it's closing. As you can tell from the long shadows in the sample shots below (not great photos, but great examples), I prefer to arrive early in the morning before the parks get busy. Others tend to stay late and get great night shots of everything all lit up - but without any people.
http://i235.*************************************DM_110311_081028.jpg
http://i235.*************************************DM_110311_081632.jpg
If you were to take the same exact photos an hour later, the streets would be packed with people.
And for those of you wishing to see photos with more people in them, below is a shot from Epcot with lots of people.
http://i235.*************************************DM_110311_111758.jpg