I am right now experiencing and dealing with the same problem. My car is a 2005 Nissan Sentra 1.8. I first changed the thermostat, but that is not necessary in many cases.
Remove the radiator cap and wrap a paper clip around the center of it, just under where the flat piece of metal retains the rubber seal. (See image below.) This will allow water and air to escape the radiator and go into the reservoir. Put a hydraulic jack under the front of your car in the center (there is a factory 'nipple' there for just this use). Jack the front of your car up until the front tires just leave the ground (best done on hard, flat surface). This will allow air to travel up the motor & head, out through the hose and exit the top of the radiator and into the reservoir. Be sure the radiator is full of anti-freeze/coolant. NOT just water, because you will have to go through all this again before winter. Place the 'altered' cap on the radiator and start the car. Let the car run until the temp comes up to normal operating temperature. The coolant should now fill the reservoir. Now, turn the car off and do nothing else, for now. Wait until the motor is cool and the coolant in the reservoir should go back into the radiator. DO NOT allow the reservoir to get empty, or air will enter again and you will have to repeat all this again! If you keep enough coolant in the reservoir tank, the cooling motor will draw pure coolant/anti-freeze back, leaving the air out. When the motor is cool, be sure the coolant level in the reservoir is at the "Cold" or "Min" mark. Start the car again and watch the reservoir through the filler hole. You should begin to see air bubbles exiting into the tank. This is GOOD! Allow the motor to run at idle until you see no more air bubbles (probably 10 minutes or more). Add coolant as necessary as the level will probably drop a little as the engine's fan comes on, causing negative pressure in the motor/radiator. Make sure the reservoir coolant tank level is at "Hot" or Max" and turn the motor off again. After repeating this process two or three times, the heater will probably put out a "little" heat, but not a lot. If the heater begins producing max heat, your job is probably done. With the paper clip still in the radiator cap, put it out on the road, where you can get it up to speed (55 - 65 mph) and watch your temp gauge. At some point, the temp should drop down to normal and the heater work good again. The key is getting ALL the air out of the motor and head, without drawing the air back inside.
When everything is good (hopefully running cool and heater working), allow the motor to cool down (COLD) and remove the paper clip from the radiator cap.
Do this ONLY when the motor is COLD, or you could create another air pocket. Do not add coolant/anti-freeze to the radiator at this time (REALLY!). Just replace the cap and add fluid to the reservoir, if needed.
I hope this helps you.