Go Slow On A Cold Engine
A cold engine uses significantly more fuel then a warm engine. So try to either avoid short journeys completely or drive super efficiently until the engine has warmed up. Dependant on the roads you are driving on and the level of traffic you may not be able to drive effeciently at all (rush hour), but do your best as you could save a lot of money over the winter months, and if possible avoid journeys altogether.
Every engine has an optimum performance temperature. In cold weather, it takes more time for the engine to arrive at this temperature. Idling does not fix this problem. It only further decreases MPG and increases emissions. Cold cars should be gently driven until the engine temperature rises a significant amount.
Why do the problems occur?
Engine fluids and lubricants, such as oil, transmission fluid, differential fluid, and bearing grease, thicken as they cool, says the DOE (U.S. Department of Energy). The increased viscosity adds drag resistance, your engine is less effective.
You also have to remember that with the cold weather other factors affect your consumption rather then just your engine. For example slushy conditions on the road, and increased weight on your car, like snow, ice or just your winter stuff in the boot. The need for accessories such as heaters, heated seats, heated windows/mirrors also uses more fuel. And if you want to get really picky cold air is denser so you have more resistance to go through!
So to recap; try to avoid journeys when possible, and take it super easy when the car is warming up, it could save you a lot of money over the months.
Some journeys which are 2 or 3 miles can't be avoided, especially in winter. In these situations your fuel consumption will be very poor because the engine might not even get warm by the time you arrive at your destination. Also in rush hour it could be a problem taking it easy.
Image credit: sxc / ColinBroug 2010