Differences of opinion can spark great innovations. If everyone on the team always agrees with any idea that's presented, there are probably a few who lack the courage to express their true opinion. Be willing to listen to others' opinions and to state your own disagreements respectfully.
Remember that disagreeing with an idea is not necessarily disagreeing with a person. You're all on the same team with the same end goal. Avoid any personal attacks and put-downs and acknowledge any good points of the idea.
Make sure that you have good reasons to back up why you disagree. It's not helpful to simply say, “I disagree with this plan” or “This won't work”. Frame your opinion with the aims of the team in mind and explain why following this particular course may not be the best way to reach these targets.
A disagreement becomes productive when you can work through it to reach a creative solution. Before you bring it up, have at least a partial solution in mind. That way, you're making constructive suggestions, not just shutting an idea down. Expect objections to your new plan and prepare answers beforehand.
Recognise that your new solution may not be instantly accepted and be willing to accept compromise. Keep an open mind and listen to all sides. Expressing a healthy sense of disagreement doesn't mean you'll ultimately get your way, of course. Be as open to accepting feedback from others as you are to giving it. After everyone on the team has had their say, some will need to compromise and commit to working together on whatever has been decided.
Respect others' differences
Don't make it personal
Be able to back up your opinion
Offer creative solutions to problems
Be willing to compromise