The most simple method is simply to ask them to do it. That is a matter of practice. If the client says that she can't be creative, then you think up some exercises that require her to be creative and you get her to do them. If she says she can't talk about sex, then you ask her to talk about what it is she doesn't like talking about. And of course, that is still talking about it.
F: "What don't you want to talk about?"
F: "What would you rather not say about sex?"
C: "Well .. how it is done"
F: "Oh, how is it done?"
C: "Eh .. in the dark"
F: "Why is it in the dark?"
C: "Because I am a bit .. eh .. embarrassed"
F: "About what?"
C: "About .. somebody seeing my body"
Sometimes people need to be tricked. Mainly we are helping them trick their own limiting reactions long enough for them to realize that they don't need to have them. Asking the client to talk about something she doesn't feel she can talk about is a trick. It is not a very hidden trick, but she probably really wants to be tricked into doing it, so she will go along with it.
Sometimes more hidden tricks are required. If people get their fixed ideas challenged directly, they might just refuse to cooperate. But if you work around them and trick them into contradicting themselves, then they are forced to re-evaluate the situation. A client will usually find that very funny and will heartily enjoy the trick that was played on them. But all we did was to get them to do what they said they couldn't, or not do what they said they always would, or something like that.
C: "I can't get emotional"
F: "How would it affect your life if you could never, ever have emotions for as long as you live?"
C: "That would be a pretty sorry state of affairs"
F: "What would you feel about that if that was all you could look forward to?"
C: "... That is really sad" (gets emotional)
Other things that people can't do don't require any hidden tricks, but simply practice. If the client says that she can't levitate, the obvious question is: "Did you ever practice?" The idea of practicing what you want to do sometimes comes as a complete surprise.
Sometimes the mere suggestion of practice is enough to get the person going. Otherwise, you might want to do the practice with the person as a process, or you might devise some exercises she can do on her own.
C: "I don't know how to talk to my mother"
F: "Well, imagine that she is sitting in front of you"
F: "Now say something to her"
C: "Hi Mom!"
It might be a communication thing, it might be a spiritual exercise, or it might be something the person physically needs to go do. If she can't play tennis, she should maybe sign up for some lessons.
When we are talking about personal behavior and ability, the interesting thing is that a person is usually already doing whatever she would say that she can't do. There is always some context where she is already doing it. She just has herself compartmentalized so that she doesn't notice. If she says that she can't enjoy life, then of course there is a part of life somewhere that she does indeed enjoy. If she says that she can't be organized, then of course there is something somewhere that is organized in her life. Showing her the counter-example will force her to reconcile two parts of herself that she has kept from each other. And will allow her positive resources to be more generally available.
C: "I would never yell at anybody, that is a bad thing to do."
F: "What if you saw a little kid on the other side of the street about to walk out in front of a truck, what would you do?"
C: "I would yell at him to stop"
F: "So, is that bad?"
C: "Eh, I guess it depends on the intention, then."
Likewise, people are usually doing themselves whatever they accuse others of. Most criticism really says something about the accuser, not the accused.
C: "My husband is keeping secrets from me"
F: "OK. Is there something you haven't told him?"
C: "Well ... I have been putting some of the household money aside"
Another example of showing a person that she is already doing what is necessary, is the technique called Backtracking. There is something she wants to do in the future that she doesn't quite seem to get anywhere on. You ask her what she would have to do before she can do that. When she has answered that you ask her what she would have to do before she can do that. And so forth, you go backwards, getting what she needs to do before. Eventually you will get back to something she is already doing right now in present time. And then her present action she already is doing gets connected up with her future goal, and she is already moving in the desired direction.
That one can't do something is ultimately only a thought. But it is a causative thought. If you say you can't do it, you are quite right. But if you change your mind and realize that you CAN do it, then you are quite right too. It is just a matter of choice. And thoughts that are backed up by experience are a lot more persistent. So, if you have experienced that indeed you can do it, then so it is.