I want to take a brief moment point out one of my favorite and most used apps: LaunchBar from the folks at Objective Development.
I've been using it for years, mostly as a general launch utility (opening up ssh sessions, performing web searches, opening projects, looking up Cocoa APIs, I even used to point it to my project directories so I could get quick access to my source files from wherever).
But this past year, I discovered that it has a built in clipboard history which you can access with a hotkey, and it has really changed the way I program. I don't know how I used to live without a clipboard history before- having place where I can quickly store little snippets of code or variable names has been indispensable to me.
So if you aren't already using an app like LaunchBar with clipboard history (and there's a handful of them out there), give it a try for a while.
P.S., if the ObDev guys are listening- I'd love to have a simple API to add my own actions to LaunchBar, similar to what you used to be able to do with Quicksilver.