Ceramic heaters certainly help. I have to admit that the few times I've used them I was less than impressed, but then again, I admit I only let one run for an hour or so before turning it off.
NO, leaving the vent open does not mean the furnace will run continously. But that being said I can't imagine running it at 74 degrees either. You can tell I'm from a cold weather state as I run mine at about 68 degrees daytime, colder at night (thought that's what warm blankets/comforters/quilts were for). Actually, having ventilation will help the furnace to run more efficiently.