Recover files after emptying "Trash"

I just found a HELP article on "Erasing free disc space" in which it says

When you delete files by emptying the Trash, Mac OS X deletes the information used to access the files but doesn’t actually delete the files. Although the disk space used by deleted files is marked as free space, deleted files remain intact until new data is written over them. As a result, deleted files can be recovered.

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I tired two very popular programs (I avoid mentioning their names) which claimed to do the very exact thing, 'recover files that were finally deleted after emptying "Trash" - but did do nothing alike!

Since Apple claims there is a way to recover them (unless they were overwritten) my question simply is
How and With What can those files be recovered!

Thank you

Deleted files can only be recovered if you immediately stop using the computer's hard drive. The reason is since the operating system does not remove the files but releases the space to the system, that released space can be used by the system to write to. Once deleted files are overwritten, they can not be recovered. (I'm sure the CIA can do it but we don't have their resources)

Your deleted files were probably overwritten and thus can not be recovered by software. Or you were using demo software which "finds" the deleted files but doesn't recover them until you pay for the program.

Regards.