Typically, I put it as an exercise to my players -- have them come up with, beforehand, what their soul looks like. They've generally had a lot of fun coming up with the symbolism, the structure of it, etc.
For most NPCs, I come up with it on the fly, but when there's someone important, I typically come up with the gist beforehand. Once or twice I've had a soulgaze start an entirely mental scene -- typically, if the soulgaze is used as part of freeing someone from compulsion or mind control or something.