How long have you been at this plateau? Most people tend to come across this at some stage in their journey, but then it suddenly starts to take off again.
Did your Dr referred you to a Registered Dietitian along with prescribing Phentermine? If so, then it may be worth talking to him/her, or asking your Doctor for a referral to one.
Apart from that, are you weighing all of your food for increased accuracy and entering it into the Nutrition Tracker? If not, that is a real good place to start. By looking at that, it will enable you to see if there is any way that you can tweak your intake.
Apart from that, below are some links that you may find helpful: www.sparkpeople.com/resource/nutrition_art
I hope that I haven't overwhelmed you with reading - LOL!