First, verify you thermostat is working correction. If the temp gage is rising to the correct temp and holding, you thermostat should be good.
Two, make sure your car does not have a heater core shutoff valve. On my car, there is an electric one that can get stuck closed. If it does not, which is likely
Three, your heater core could be plugged up. It may need replaced.
One off chance.... did you have any work down to the system prior to the heating problem? If yes, some models can get an air bubble in the system preventing coolant from circlating to the heater core. This air must be bled out. How depends on the model. Most do not have this problem, and it should have worked it's way out of the system if you drove it at highway speeds.
My heat goes from kind of warm to cold, then sometimes just stays kind of cold depending on the temp outside the car.
Here is what I have done so far:
Change water pump due to noise it was making.
Changed thermostat
Changed temp sensor
Flushed radiator
I have also checked under the glove box to see if the blend doors where maybe bad. Once I take the motor off of the blend door I can hear the doors inside move from side to side. I don?t know if the both are moving but I can hear something moving. I don?t have a clue what my problem could be.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic