I believe the "trick" is to get them to see that they can let loose when they're alone with you in the bed... assume they have anything to let loose.
My gf was hella sigh when I first met her. As things progressed and we took it to another (sexual) level, she didn't even want me to look at her vagina, let alone go down on her. I'd constantly let her know how perfect she was and how I wanted to please her in every way possible, and eventually she opened up (physically) and began to reciprocate.
From there it was just a matter of getting her hot enough to try anything. I'd start by asking her questions like "Is it too deep" or "does it feel good." She'd only respond with a simple "yes" or "mmhmm" but that was a start.
Once she had no problem voicing that I started teasing her. I'd ask "how bad do you want it" when I was getting her going through oral, though I could tell she wanted me in her. She would buck up and respond in a coy manner, moving from one sense of the word to another.
Soon after doing things like this, she would have no problem voicing what she wanted. I then moved to asking things that would cause her to answer with HOW she wanted it. From there, she just started taking the initiative. I'm sure she could tell that it turned me on. She ended up saying all kinds of naughty stuff when she's horny.
All this said, I hope you'll realize that you should avoid such things if she's genuinely uncomfortable with it. If you can't tell the difference between whether she just needs to open up or if she's just not into it, you should probably leave it alone.
