Travel to Civita di Bagnoregio from Orvieto

A car. I hate to say it, but a car is the best. The bus times, in my experience, do not support day trippers very well. Here is a place you can research the COTRAL bus schedule.

There is a special bus that runs Spring-Fall but that won't help you. Best bet in February is a car.

BTW we did this trip from Roma one February on a clear, crisp day (a Sunday) and it was marvelous. Maybe rent a car for just the one day and take in some other local sites as well.

Rental car is always the most flexible plan. Otherwise, there's a Cotral bus to Bagnoregio (does not run Sundays or Holidays), well described in Rick's Italy book (but confirm schedules at the Tourist Info office). I had no luck with the Cotral web site today, but don't let that throw you off. Plan to walk 20-30 minutes from the bus stop to the hill town, since the local shuttle has limited service in the winter, if it runs at all.

I took the bus without difficulty, but my visit was in June.

Should you decide to rent a car, there are a couple of other places you could visit that day: Bolsena and Montefiascone. I'm not sure about the parking situation, though.