I think the best way is to regularly update your will. Especially if you expect your estate to be over $1M excluding primary residence.
Well, you should update your will if your priorities change. For all else, you can specify what you want using "if then else" clauses. Considering the over $1M estate, I'd expect the majority of active LW'ers to currently or eventually (at the time of death) fall in this bracket.
EDIT: The following is a joke: (I'm active because of that expectation, don't ruin it o.o)
Bryan Caplan of Econlog asks his readers how to improve his will (given a few constraints) in light of the principles of optimal philanthropy. His current draft reads:
The full blog post can be found here.
Robin Hanson responds: