Run AppleScript Editor and copy and paste the following code. You'll have to change the path to match your username using AppleScript Editor. Next, save it as an application named 'Clear Downloads' into your Downloads folder. Feel free to add a custom icon if you wish. To test, just click your downloads stack on the dock and select 'Clear Downloads.'
Here's the script:
try
tell application "Finder"
move (files of alias "Macintosh HD:Users:YOURUSERNAMEGOESHERE:Downloads" whose name is not "Clear Downloads.app") to trash
end tell
on error -- oops, couldn't move the file
display dialog ("Error") buttons {"OK"}
end try
[crarko adds: I debated publishing this one, but I think the basic idea is a sound one. I've used a similar script to create a Trash item on my desktop, and then attached the script as a Folder Action to move what I put in it to the actual (hidden) Trash folder.]

