This is probably a well known tip, but I discovered it today and it is totally invaluable!
When you're copying a large number of files from one place to another (as I tend to do when backing up photos), you're presented with this option:
For simplicity, you can just overwrite all of the existing photos with an exact copy, which really is just a waste of time. What's even more of a waste of time though is sitting there for 20 minutes clicking "No" for every file so that you only copy over unique files.
Here's the tip: hold down Shift when you click no. It's essentially a virtual button that says "No to all" on it. Frikkin' sweet.
Thanks to Jigar Mehta who happened to be the first link I clicked on.