How much you want to spend. cheapest way is to get a car heater from Fry's for about $12.
otherwise, check the pressure in the water line that splits to the heat exchanger( hot coolant goes through it, while fan blows air through it to heat the car). the exchanger element could be clogged, hose plugged. you may have to clean them both or replace the ones that are too far gone.
good luck
