When we talk about visualization most people are going to think of things like reading scenarios of walking through the forest or laying on the beach listening to waves. That can be a visualization exercise. However, what works best is usually something personal.
Many people tell me that they enjoy reliving a positive experience, a date, a vacation, their wedding. This is done by the retelling of the story by your partner. Being sure to include all of the details to actually help remember, like sights, smells, tastes, sounds. Utilizing all of the senses is important.
Other people find this to be annoying and use visualization to produce a scene that they desire, for instance their ideal birth. This is actually a personal favorite of mine. My husband will get me into a relaxing position, spend a few minutes checking for tension and then begin to tell me the birth of the baby. I can then process the birth and he can observe for tension. Did I tense when he said anything particular? Sometimes I notice and sometimes I don't. If it's something I have control over we can pin point that as an issue to deal with.
Explaining what is going on in the body and using those images as a relaxation tool is also beneficial. For some woman knowing that what she is feeling is her cervix opening, and giving her an image of a baby help to dilate the cervix or of an abstract object like a flower bud opening often work well.
Some women will even choose a single, inanimate object. It might be a photo, a special relaxation card, solid sheet of paper, basically whatever works.