Use a tool like Disk Inventory X or DaisyDisk, point it at ~/Library/Application Support and nuke anything with more that X MB (I'd recommend 10) you don't recognize or no longer use. Keep the files around another month in case you start an application and have an unexpected first-launch experience. Simply start any application once you install. Anything unused in the last X weeks/months can go. Go to ~/Library/Preferences/ and sort by modification date.
