First, security commitments are serious things; don't make them unless you have the support, desire, and means to follow through.
Second, Russia is ruled by some nasty people these days, but they are not Hitler or Stalin, and they can't be expected to tolerate direct challenges from their border any more than an American president could from, say, Cuba. (This is not to draw any moral equations, only to point out basic facts.)
Third, the sad truth is that—in part because the Cold War is over, in part because skyrocketing oil prices have engorged the Russians' coffers—we have very little leverage over what the Russians do, at least in what they see as their own security sphere. And our top officials only announce this fact loud and clear when they issue ultimatums that go ignored without consequences.
In the short term, if an independent Georgia is worth saving, the Russians need some assurances—for instance, a pledge that Georgia won't be admitted into NATO or the European Union—in exchange for keeping the country and its elected government intact. (Those who consider this "appeasement" are invited to submit other ideas that don't lead either to Georgia's utter dismantlement or to a major war.)
If a newly expansive Russia is worth worrying about (and maybe it is), then it's time to bring back Washington-Moscow summitry. Relations have soured so intensely in recent years and over such peripheral issues (such as basing a useless missile-defense system in the Czech Republic) that a new president—not just his secretary of state, but the president himself—could do worse than sit down with Medvedev and/or Putin, if just to lay out issues of agreement and disagreement and then go from there. It's staggering that no such talks have taken place so far this century.
In the long term, the best way to take Russia down a notch (along with Iran, Venezuela, and other hostile powers overflowing with oil money) is to pursue policies and fund technologies that slash the demand for oil. The Georgia crisis should make clear, if it isn't already, that this is a matter of hard-headed national security.