If an air system, check the filter.
If a water/baseboard, maybe clean the fins.
Check the batteries in your programmable thermostat - get one if you don't have one already.
Tune up your furnace/boiler.
Unpack the blankets, and sweaters, and slippers.
(Please note: when you choose an answer as a Best Answer, or at least give a thumbs up, it helps those who answer questions here.)
Look for cracks around the house and fix them.
Buying one of those portable heaters help (very inexpensive to buy)
Do a walk though of the apartment and see how you could isolate it.
If you are on the sunny side make sure you don't blind the apartment with curtains/sliders.
Hope this helps.
1. Make sure heating system gets serviced including hot water heater and pipes are properly insulated.
2. Make sure that windows are working properly and that the seals are not broken.
3. Make sure that any place air comes in gets sealed. You can buy winterizing supplies in Home Depot for very cheap money
4. Make sure you have an extra space heater around in case system fails and also have candles and blankets ready in case power goes out.
5. Lastly keep important phone numbers handy for management company, plumbers, electricians, city hall in case all the above fails.