Here's another vote for the Nurtured Heart approach. In our experience it works best if you go the whole hog-- implementing the point system and everything-- instead of just skimming it for words of gentle wisdom and trying to cut down on the contentiousness. The book itself (the original, not the workbook that we don't have) is a little too new-age for our tastes, and wastes a lot of pages restating things, but the methods do work whether you agree with all the ideas or not.
Another book I can recommend in a similar vein is "Beyond Time-Out: From Chaos to Calm", by Beth A. Grosshans and Janet H. Burton. It has ideas on how to remedy power imbalances within the family, which I suspect are more likely to happen with gifted kids, and why they occur, which may be enlightening even if you just go with the Nurtured Heart training methods.
