I don't!
I don't see the point in reuseing a DV tape when they can now be bought for around £3 each. Say you record something you want to keep, copy it to DVD-R or even VHS and wipe the DV tape. If you then loose the recording or it breaks/gets damaged you then have no option to recover it. I still have the origional DV tapes for everything I have filmed.
But if I did ever need to reuse a tape I would bring it into work and use a machine that we have for erasing data from old tapes. This passes a strong magnet over the tape and wipes anything recorded. Any strong magnet will do this like a TV or speaker.
You could just record over the old recording, but this often does not completely wipe it and you might be able to see the old recording as ghosting. This all depends on how good the heads on the cam are and the quality & age of the DV tape.
Mark.