Over the years as I've read my Bible I have come to look at passages like this as coming as close as you will find to the subject of employee/employer relationships. Though sometimes certain employees feel they are treated like slaves, the point Paul is making is that a Christian servant, whether slave or employee, is given instruction by God as to how to live a Christian life in their work. This relates back to husbands and wives and their proper relationship with one another. How? A man or woman working outside the home to provide for their family will find their employment enhanced because of a good relationship with their spouse in their home. Employment can bring enough stress on its own without the added stress of a rocky marriage.
Paul encourages believers to serve their masters (employers) as if the employer was Christ Himself. That simply means that a Christian will do their best for the one they work for as they would do their best for Christ. Showing respect and "fear" for your employer builds a good relationship with them. Respect is received when respect is given. Showing respect for your employer gives him/her the natural desire to reciprocate that respect. Serving them with sincerity helps your employer know you're reliable and willing to give an honest day's work for an honest day's pay. Even if you're not getting words of appreciation from your employer try to remember that you're being watched by your Savior and He is the one who will give you His "well done" when you stand in His presence.