MacRumors and other sites will provide instructions on how to create a USB install drive to do a clean install once Yosemite is released Thursday after Apple's event.
Essentially, you'll download Yosemite via the App Store, create a USB install drive by running a script via Terminal, reboot holding the Option key and boot to the USB install drive, delete the partition, rename to whatever you want (stick with Macintosh HD) and install from there.