Two suggestions on being missionaries in the workplace, from In His Image by Mike Wilkins:
First, we see our work as a means to an end, not the end in itself (Luke 12:16-21, Ephesians 4:28). Many Christians get caught up in their work and forget that we have higher purposes for being here. Sometimes we become so focused on the job that we never even consider how it can be a means to bringing light and salt into the working situation.
Second, our work is part of our commission from God to care for His creation (Genesis 1:28). Work is part of our calling in life. Whatever work we have at any particular time in our lives is an opportunity from God. Do you take pride in your work, no matter what it is? One of the most powerful witnesses is to be a good worker.