It may be hard to pass up the shiny signs advertising low prices in December, but putting off the purchase until January or February could bring an even better deal, Shattuck says. "Sales often extend into January, and since it's typically a slow month, dealers are more likely to negotiate."

Used-car prices start to rise again in January. If you plan to trade in your car, you could get more for your vehicle at that time. Good incentives can even be found in February, as it's traditionally the slowest month for dealers.

The price of certain cars fluctuates as the seasons change, Rabkin says. If a convertible is on your wish list this year, you may want to negotiate for a better price from the dealer while the weather is cold and snowy. You'll have more leverage than you would during the summer months.