Leonard, I tend to agree with most of the other answers here, upgrade your furnace,service it and keep the paperwork/reciepts tape them to your wall near the furnace so that when buyers come they see your on top of things it relaxes the buyer to know you've put effort into the home.
As a note there are other things you can do which i'm sure your Realtor has told you, however I'll mention a few here just in case.
Bathrooms, always clean take your personl items away put them in a closet cabinet or box.
Bedrooms put away anything personal, pictures,perfumes on dressers etc. doing this lets the buyers picture the room as their own and not yours.
Paint where you can basic colors tan/lite coffee colors seem to be the style. White isn't as popular and tends to let the buyer think it was a quick job. So do alittle different in the color dept.
basements, clean dust free, if it's finished then no problem, if not store your personals etc in one corner and leave the rest open.
Smell the ol baking cookies is always nice and inviting. Don't over kill it but make sure a fresh smell is in the air. Running the dryer with dryer sheets is always good so if you get a call to show your property do some laundry before showing.
Remember to take away extra furniture or pictures that give the room that full feeling, less is more in selling your home.
I tell you these things because sometimes putting money into things such as furnaces, is a bit costly and may not have the return your looking for. Listed above is things that only cost you sweat. and is very helpful in selling your home and putting it in the best possible light.
just as note if you have a lawn, clean and solar lighting is a nice low cost touch for curb appeal.
All the best to you!