Provided you don't actually still have the installer somewhere on your mac, you should be able to redownload it from the purchases tab in the mac app store. You have to sign in though to see the purchases first.
Of course if you still have the installer somewhere the redownload is not necessary. The first thing I do after downloading a full installer is to back it up somewhere so that I never (or rarely ever) have to redownload it.
Where is the installer located after you download it from the App Store? Reason I ask is because I am having huge difficulties at the moment after my install (see my thread lol) and need to figure something out; maybe a clean install is the way to go for me. But the question I have is, I have downloaded and installed the OS X Beta, yet when I reopen the App Store, after the download and install, the OS X file is still listed under purchases and says "Download" and not "Install" for some reason. So does that mean that there is no local copy of that file when it was downloaded? I would also like to mount a USB and do a clean install but need the file and as the user posted, my Dev account says that I already used the code.