Keep it well insulated if possible, seal drafty windows with plastic sheeting, keep the door closed, and use an oil radiator to heat that room specifically. That's what I've done for the last several years without too much of a spike in my utility bills (depending on the apartment at the time). I currently only need my radiator on the nearly lowest setting to keep the room at about 73-76 with my HVAC set to 70 for the rest of the apt.
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