My kids are only 3.5 and 7m, but things are heating up (well, mostly for the older one) already. I just finished _Siblings w/o Rivalry_ and it's *great*. Definitely a keeper. While the things she puts forth are fairly straightforward, i can see how it would take constant vigilance to keep them in practice. Some of the things don't apply to my situation yet bc of their ages, but the things i have done have made an improvement. For example, i always acknowlege what i think is going on with my older one. If she is freaking out over ds coming for her blocks, i'll say, " wow, dd, looks like you really dont' want ds touching your blocks." and she'll say something like, "yeah, they're mine!" and maybe a bit more. and then depending on the situation, i'll either just scoop ds up and take him away for a bit, or i'll ask her how she thinks we can solve the problem. ususally she comes up with another toy, or i try and suggest giving him 1 or 2 blocks. its different each time, but it addresses her frustration.
i am sure it is much more difficult w/ older children, but the book does a great job of giving examples. its an easy read- illustrations and stuff. written by the same folks who wrote, _learning to talk so kids will listen and listen so kids will talk_
sorry if you've already read this and its redundant!