How to write salary increase letter to get positive response

Hi! i had a difficulty in writing salary increase since my employer are very kind but in the 2 years i've worked for the company i dont get a salary increase and still i've paid below minimum industry averages.They gave me more duties and responsibilities without increasing my salary.
Being a multi-task employee is a difficult one.i've handled purchases,letter/quotations,& some administrative works, doing artworks & some jobs of my co-employees if she/he's on leave.
Hope you would help me and give sample letter.

Thanks!