Retrieve data after deleting from Trash in OS X
If you have used Time Machine this will be rather easy:

Go to the Applications folder and open the Time Machine application. Browse to where the file used to live, go to previous backups until it appears, then choose "Restore" from the right-click menu.

Otherwise, if you are still using the same computer, then you really are in trouble. The chances of recovery with tools like Data Rescue go down every minute the machine is in use.