Obviously, the way you support the troops is contingent upon your analysis of the war. If you think the war is succeeding, then supporting the war is a way of supporting the troops. If you think the war is doomed to failure, though, proposing that more troops die in vain is not a way of supporting them.Now that I think about it, though, even this presents difficulties. Telling the troops that they're dying in vain is demoralizing. Saying a war is "doomed" to failure is demoralizing. It means you're losing. The soldiers don't want to think they're losing. Losing is not an option.
So, I guess you need even more nuance. "We, your political leaders sent you into a war that didn't have a chance right from the start. Sorry. All the shame and blame belongs to us. Now the best we can do is get you out of there as soon as possible."
Update Aug.31/07 11:05am: And in a related item, Glenn Greenwald at Salon.com discusses how even political journalists cannot seem to disentangle the concept of "supporting the troops" from the idea of continuing the war.