If you're using the OS X Terminal app (as stated by the OP), a better approach (thanks to Chris Page's answer to How do I reset the scrollback in the terminal via a shell command?) is just this:
clear && printf '\e[3J'
or more concisely (hat tip to user qiuyi):
printf '\33c\e[3J'
which clears the scrollback buffer as well as the screen. There are other options as well. See Chris Page's answer to How do I reset the scrollback in the terminal via a shell command? for more information.
The AppleScript answer given in this thread works, but it has the nasty side effect of clearing any terminal window that happens to be active. This is surprising if you're running the script in one window and trying to get work done in another!
You avoid this by refining the AppleScript to only clear the screen if it is frontmost by doing this (taken from MattiSG's answer to How do I reset the scrollback in the terminal via a shell command?):
osascript -e 'if application "Terminal" is frontmost then tell application "System Events" to keystroke "k" using command down'
... but as when it's not the current window, the output will stack up until it becomes current again, which probably isn't what you want.
On Mac OS X Terminal, this functionality is already built in to the Terminal Application as menu View → Clear Scrollback (the default is CMD + K).
So you can re-assign this as you like with Apple's Keyboard shortcuts. Just add a new shortcut for Terminal with the command "Clear Scrollback". (I use CMD + L, because it's similar to Ctrl + L to clear the current screen contents, without clearing the buffer.)
I am not sure how you would use this in a script (maybe AppleScript as others have pointed out).
Or you can send a page break (ASCII form feed) by pressing Ctrl + L.
While this technically just starts a new page, this has the same net effect as all the other methods, while being a lot faster (except for the Apple + K solution, of course).
And because this is an ASCII control command, and it works in all shells.
Command+K. – eonil