How do I delete a single file from the Trash?
The Trash isn't intended for files you may want to delete later but rather as a safety net if you delete the wrong files. If you want to mark files for later deletion, either use a dedicated folder (as already mentioned in another comment) or mark them with a specific label/color.

To delete individual files from the Trash


Open the trash in the Finder
Open Terminal.app
Type rm -rf and then one space character at the end
Drag one or more files to be deleted into the Terminal window ('til you see the green + sign)
Press Enter in Terminal.app
Puff, files are gone!


If you need this very often, I would recommend writing a service using Automator or getting more familiar with Terminal.app/bash.