The first thing to do is to immediately stop using the drive that the files were on. If they were on the boot drive, then it would be best to find another computer to use until you have recovered those files. Reason being that the longer you use the drive that the files were on, the greater the chance that the sectors on the drive that contained those files will be overwritten with new data. Even if you don't read or write to the drive, OS X does a lot of reading and writing behind-the-scenes and can write data over the area on the disk that the files were on.
ProSoft's Data Rescue X can help. Do a search for it on versiontracker.com.