Honesty | How to say 'no' at work without making enemies

Your colleagues and manager have presented some new ideas and stratagems for the company moving forward.

Alas, they all sound terrible and you’re absolutely sure they won’t work – they didn’t in your previous company after all and the strategies just sound like diagnoses, not solutions. But how do you say 'no'? How do you approach the situation without sounding like you’re personally attacking the credibility of someone’s work?