First of all get proper training. Reading it on the web isn't. I have seen some really bad practises on the web.

I know the dulfersitz, but don't really trust it or feel comfortable with. Apparently it is now recommended to wear a harness with a prussik on it, to back up the dulfersitz. Which kind of defeats the use of the dulfersitz...

I'm a sport climber, alpine climber and also do a little industrial climbing. I personally only rappel with proper safety equipment. Not much point in risking my life to save carring a few extra gramms of equipment. Besides you can get a harness, HMS biner and a piece of prussik for less than 40 bucks and it weights not much at all.

There are a few really know mistakes with rappeling:
- Rappeling of the end of the rope: Tie a end knot, big enough to get stuck in the belay device and with plenty of tail.
- Letting go of the rope (To much weight, not enough friction, trying to free the rope, etc.): Make sure you have a back up.
- Anchor failure: find proper anchors!
- Rope abrasion: check how the rope is hanging and use rope protectors if needed.
- Figure of eights braking carabiners: Allign it properly or use something else.