What is the best way to ask my doctor for a referral to a psychologist?

Describe symptoms. Describe your symptoms to your doctor and ask him/her if a referral is appropriate. He/she may recommend medical tests first to rule out a non-psychiatric condition.
Blunt and honest. I would just be blunt and honest about your issues and ask for a referral. If for some reason you cannot obtain one, you can call your local hospital and see if they have anyone on staff.
Openly and honestly. Simply ask. The physician is there to serve your best interest. If you feel that counseling with a psychologist would benefit you, simply express that to your physician in confidence. You may feel uncomfortable bringing it up, but remember that we deal with these topics daily and nothing ask is going to surprise us.
Symptoms. Describe the symptoms you are experiencing to your doctor. Ask him or her for a referral to a reputable psychologist. If the symptoms are severe, also ask your doctor for referral for a reputable psychiatrist who can do a complete evaluation, with possible suggestions for medication.