Illustrator is a strange piece of software, full of not-very-obvious features and counter-intuitive processes. I've met people who've used the application for years and were unaware of tricks and techniques I thought everyone knew…! By the same token, I've talked to people who've only been using Illustrator for a relatively short time who knew things that had passed me by completely…
So… I'm going to post an Illustrator tip every few days until I run out of things to say! Feel free to ask questions or offer tips of your own in the comments. I'm running CS6, so also feel free to correct me in the comments if any tip I come up with works differently (or doesn't work at all) in different versions.
ALT (option) DRAG any object, or group of objects to create a duplicate. The layer(s) of the original item(s) will be preserved in the new versions.
Why not just copy and paste?
Because ALT-DRAG doesn't use the clipboard. If, for the sake of argument, you already have the text you want to use copied to the clipboard and you want to duplicate a balloon and text box so you can just paste the text in, this is the method for you.
Also, copy and paste just dumps a duplicate slightly offset from your original, which you then still have to drag to where you want it. ALT-DRAG eliminates the need for the copy/paste, incorporating the copying into the same action as positioning the new element.
I keep forgetting to hold down the ALT key before I drag…!
You can add ALT to the drag operation at any time right up until you release the mouse button to end the dragging. By the same token, if you change your mind and release ALT at any point up until you release the mouse button, the operation will revert to a normal 'move' instead of a copy.