This article from Lifehacker I found quite interesting, though I only applied a few of the tricks myself. The two tricks from the article I applied were the ability to view the contents of compressed folders and normal folders in QuickLook.
I underutilise QuickLook myself, and these should allow me to use it more than I currently do. The instructions for both are the same: download the QuickLook Generator file from the website and paste it under ~/Library/QuickLook and restart Finder. (The easiest way to restart finder is to hold Alt, right-click the Finder icon in the dock and select "relaunch."
For viewing compressed folders, download the file here.
For viewing normal folders, download the file here.
The article is great and definitely worth a read.