When I am talking dirty to my slave it is to enhance the effect that I am striving for in my interaction. Talking dirty is then all about what I want and I don't have to be a mind reader to know that. I have great confidence when the world revolves around me.
When I am spanking her and want her to get driven to a sexual frenzy, I simply tell her the things I am thinking of doing to her once I stop. Since she gets great pleasure from being used by me and bringing me pleasure, telling her how she is going to do that emphasizes the pleasure that she is getting from the spanking.
When I am wanting her to be pliable, submissive, and soft, I talk to her about how she is a good girl and how pleased I am with her service to me and how much she can take for my pleasure. This combined with the spanking releases any hold she has on self desire and puts her in a very submissive state of mind.
When I want her to work hard a pleasing me all I have to tell her is how dirty she is. I call her names while I spank her, let her know that her place is to please me, and it is easy for her to release all of her inhibitions and focus on my pleasure.
I may not be that great at dirty talk but I know how to get what I want. Many people are insecure with this and afraid that they might not do it right or they might say the wrong thing. If a person just says what they feel, desire, or enjoy, they are on the right track. Sometimes it might not get the effect that you are shooting for so you can think of adjusting it next time. However, sometimes you just might get the result you want. Decide what effect you want and then think about what your partner would have to do or be to accentuate that effect. Try describing what you will do to enhance what you seek. Come from your personal desire without worry about your partner and see where this leads.