How do I asked for a new title and salary?

I was hired at my current job approximately 8 years ago as a medical transcriptionist. Since that time, we've added two scanners, several radiologists and three transcriptionists. I currently "oversee" the transcription department as well as the three transcriptionists, work directly with all of the radiologists and have the respect of all involved. I've been given all of the responsibilities of a supervisor but not the title "supervisor" nor the salary. I'm still getting paid hourly but feel I deserve the title and pay. How should I go about approaching my director. I've been told repeatedly that I am a very valuable to this company, but I'm not sure of the correct way to go about asking for what I want. I've checked the standard salary for a transcription supervisor in my area, and am being severely underpaid for the work that I am doing. Any input would be greatly appreciated.