There are three main reasons why a woman would be "stiff". Either 1) she's not interested in being with you, 2) she's not comfortable in the situation or 3) she's inexperienced. Hopefully she is into you so we can ignore #1. For the other two, you have to make sure that she's comfortable first and foremost. It is not rocket science; the best way to make her comfortable is to know what she likes. Ask her beforehand (not right before the deed) what kinds of things she likes in the bedroom. You can both discuss your likes and dislikes. She'll most likely feel closer to you after discussing those things. Once you know what she likes, you have to set the mood for her. Depending on how long you've been with this woman and what she's like, anything from a romantic candlelit dinner to washing the dishes (not kidding!) could be a turn on for her. I know, women are complicated to men but the same is true for us women about men. No one is open and shut like a book so take your time and get to know your woman!