Getting assertive without getting vicious is the most straightforward path to having your voice heard at work. Chrissy Scivicque relates how to do that in an article for U.S. News and World Report.
Strike with an Open Palm
First, know in advance what you want to say, or else you will just become the office loudmouth. Get to the bottom line as concisely as possible, and really think about and practice your phrasing to come up with the most streamlined argument possible. Remember to speak at a confident volume—not quiet, but not yelling. Likewise, be mindful not to use “devaluing” language like “just” or “I could be wrong, but…” because it needlessly weakens your argument from the get-go. At the end of the day, this all just boils down to feeling confident. Some people have it in spades; some people might want to see a hypnotist about it. You can read Scivicque’s full article here: http://money.usnews.com/money/blogs/outside-voices-careers/2013/06/20/how-to-be-assertive-and-get-what-you-want-at-work