This is a very important note to make.
Synesthesia is NOT elicited. It occurs involuntarily.
It is also "durable", which means that it doesn't change over time. A person will always (or nearly always) see the same color, or shape, or have the same impression of smell, etc to a specific stimulus.
A person may see a green, for instance, upon hearing a specific note of music, and years later have the same sense impression to that note.
It is also "non-specific", meaning a person may see green and red swirls, but not a landscape of a church, for instance.
That said, certain emotions, levels of attentiveness, and drugs (including alcohol) can effect the synesthetic experience.
Cytowic is the big name to know in synesthesia research if you want to know more, he has a few books out.
I have visual/kinesthetic -> sound synesthesia. Movements, flashing lights, or anything "phasic" (goes back and forth, on or off, round and round) gives me an impression of sound. I also hear similar noises in movement, depending on what point I focus on.