If I had a Client with who has difficulties to refuse, first I would teach him or her to relax, and then rehearse in mind saying plain firm No, while retaining the state of pleasant muscle relaxation.
I would encourage my client to see the situation the more and more clearly and realistically, and would suggest a homework to continue this mental rehearsal. BTW, a performed homework enitles the Client to a 1000 discount!
I would go with my Client through a variety of situations, starting with the easier ones, and consider the work done when the client can realistically imagine saying firm NO to a scary dominant person, and also feel that he or she can do it in a real life. This technique works very well. It's name is Image Therapy.
It is important for "No" to be straight and firm for the following reasons:
1. Some people wouldn't take it for No otherwise.
2. It is a vital social competence, the ability to say No, and my client is much better having this ability instead of palliatives of diplomatic or feminine denials like ... maybe one day... Which doesn't prevent us to work on diplomatic denials, if such a need comes.