never get too worked up

This really applies to disagreeing for specific valid reasons in almost all situations. It requires that the person you are disagreeing with be willing to engage you in discussion on the topic, and assumes they are not completely closed minded.

[edit] Steps

Start out by having at least one specific, valid & provable reason why you know this idea won't work.
Provide a suggestion or alternative idea and why.
Begin asking the person questions to help you understand their views on the idea you disagree with. eg: "for this plan, you are suggesting x?"
When they answer, follow up with another question which leads in the direction of your reason for disagreeing. eg: "x requires y, how will we make y happen?"
When you get to the point where you believe failure will occur, ask a final question eg: "y is square, but x requires round - how do we get around this?" Don't demand an answer - let the person think about it.

[edit] Tips

Liberal use of 'we' is helpful, but don't overdo it. If something is clearly your opinion use 'I'.
Be willing to consider the possibility that you are wrong.
The goal is to make the person reach the same conclusion on their own. Don't expect this to happen immediately - give it some time.
Doesn't matter what, never try to prove that you are better than the boss.
If an idea doesn't work tell them you'd like to try a different approach. Don't make it seem as if they are wrong. Just add to the problem solving arsenal.
Simply tell them you disagree. Have a conversation and see where it goes. You may not know of experiences your boss has had or they may not understand your thinking process. Adults and professionals disagree everyday. Some of the best ideas come from brainstorming and disagreements.
Remember, ultimately they are the boss. If you are specifically told to do or not do something a certain way then you should respect the authority. You never know where they are getting their cues or what demands upper management could be making of them.

[edit] Warnings

"It hasn't worked before," is not a specific, valid, and provable reason.
If you must implement the idea anyway, work your hardest to make it succeed. No one was ever hired to make their boss look bad! If it works, you're a hero for working hard, if it doesn't, no one can blame you for not trying.