With hard drives it's easy - use dban like suggested by Daemon.
With cellphones might be a bit tricky, I don't know of any tools to wipe internal memory but you could throw that shit into the microwave and cook it a bit. Remove SD card if present and crack it in half.
Phone should be pretty unusable and unreadable after a minute in the microwave... but I'm not sure... maybe techb could shed some light on how to effectively destroy memory chips.