Go Slow On A Cold Engine
A cold engine uses twice as much fuel as a warm engine, and catalytic converters take 6 miles to become effective. So 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, if possible avoid the journey completely.
Problems
Some journeys which are 2 or 3 miles can't be avoided, especially in winter. In these situations your fuel consumption will be terrible because the engine might not even get warm by the time you arrive at your destination.
