As part of being a programmer, you often are asked to provide estimates/ make slideware / do technical demos for Sales teams to present to end-clients. Sometimes we go along for the 'technical' discussions or 'strategic capability planning' or some similar mumbo-jumbo.

Sometimes, you kind of know which ones are totally going to fail and are not worth pursuing but the Sales guys present fake optimism and extract 'few more slides' out of you or the 'last conference call'. These don't lead to anywhere and are just a waste of time from other tasks for the week.

My question is how do you get out of these situations without coming across as non-cooperative.

Updated after Kate Gregory's answer:

The problem is related to projects we know are doomed (from the technical feedback we've received)

But Sales ain't convinced since they've just had a call higher up the management chain - so it's definitely going ahead !