I use padded bike underwear (like this one: http://www.loeffler.at/de/Produkte/U...ERHOSE-ELASTIC) the one shown in the picture is for men, but there is a womens version too. They have a thin but comfortable pad.

I would also second the recommendation to swap the saddle (saddle and seatpost) before riding. This would only take you a minute to set up your bike and would save you so much pain and discofort.
There are different seatpost diameters, but it's easy enough to find the right size. After work you can just put the original saddle and seatpost back on.