My sub does the fantasy exploration that ducktig flicka described, focusing on oral sex usually because the imagery of that is so strong for me.
I think the secret is to really drag it out
Go slow and repeat images. Tone of voice is important.
Speaking of which, she likes to listen to my voice. And when I run out of ideas I find a story, either here or on Literotica, that I think she'll identify with, and just read it to her slowly.
She is very willing to let me hear her arousal as I read, and I will occasionally stop and talk directly to her, adding in a few sentences of what I'd like to be doing to her specifically and, when she is about to come, I stop and tell her to come and talk dirty to her .... she seems to like the whole thing. In fact, I think I'm going to have her do the story thing for me sometime soon since it seems so successful for her.
All of this is much better if you have unlimited long distance, by the way.
her_Joe