Put them somewhere cool and dry but frost-free.
Only give them enough water to stop them shrivelling up.
Give them plenty of ventilation on dry days to prevent mildew forming.
Take off any dead leaves and flowers.

I keep mine in our rather large bathroom :o)

I never had success doing this. I just buy new plants each year. Last year they were 3/$5.00

I brought mine in from the garden and potted them up in the unheated leanto where they are standing on the windowsill. They are in full bloom and cheering up this dull season. In a few weeks I'll take cuttings and make new plants for 2016. I have done this now for 3 years and it works well.

In the winter its more likely to be damp than cold that kills them when they aren't actively growing.