Forecasting the aurora – how do you do it?

Many people ask me how they can forecast the possibilities of aurora. Sometimes they want to know if there will be auroral activity tonight, sometimes if there will be any aurora the coming weekend. And sometimes they want to know if there will be good chances for aurora during their planned vacation to a place where there usually can be aurora visible – sometimes months before they plan to travel. Can the aurora be forecasted with any vertainty? If so – how long in advance and with what kind of precision?