When you find yourself disagreeing with someone, you might find you get what you want more often if you focus on what you can agree on first and save what you don't agree on for later in the conversation.
Very few arguments occur where both parties take completely diametric points of view. That being said, it is important to find the similarities between both parties as quickly as possible and make them known in order to keep (or shift) the conversation to a more collaborative position.
Once you have made the similarities in your points known you can then start focusing on the areas where you disagree. For especially tricky issues, the larger gap that you put between agreeing and then disagreeing with someone, the better chance you have of your argument being well received.
This strategy is especially effective when you end up disagreeing with someone you don't typically get along with or a person that has a reputation for being especially argumentative.
As an added bonus, try substituting "but" for "and" in order to more effectively get your point across. "I agree that your idea will allow us to ship sooner and I think our users will have trouble understanding how the new functionality works. Do you have any ideas of how we might mitigate this?"
Disagreements are a constant factor when two or more people attempt to accomplish a goal. The better you can navigate these treacherous waters, the more effective you will be at work and in your personal life.