Trying to build influence through words is useless. If you're going to build influence in the workplace, you need to speak through your actions, or at the very least have the actions and history to back up whatever it is you're saying.

There's a difference between being assertive and being aggressive. 

You'll need to present your thoughts and ideas with a high degree of confidence, indicating your convictions, but any excessive degree of confidence could be mistaken for needless arrogance, which will compromise you...

Inconsistency is the fastest way to ruin your reputation. Consistency, on the other hand, is slow but sure: if you execute your tasks effectively and on time, day after day, eventually people will come to rely on you.

Being too stringent or adamant in your beliefs will work against you.

Work actively to show your flexibility while holding firm on your beliefs. Negotiations and compromises are often the best ways to do this.

Listen to everyone's opinion, and encourage people to speak up, especially if they don't often voice their opinions. Take time to respect and acknowledge everybody's opinion, and let people know that you value them.

Influence is most often and most easily carried through trust: only when a co-worker trusts you will he or she be open to your influence.

The easiest way to do that is to be honest, no matter what. State your opinions, disclose your apprehensions, and don't keep secrets.