This blog is on learning synesthesia ("mixing of the senses"), as a follow-up to a Yahoo question.
http://uk.answers.yahoo.com/question/index;_ylt=AnL3uRGtu8P.zVtxaoo_G2BJBgx.?qid=20070502125359AAKeppm
I know synesthesia can be strengthened, by the means described below. I expect that it can also be built in people with no previous synesthetic experience, but so far as I know that is new territory.
For anyone interested in making the attempt, it's like falling asleep: you can't command it, you can only surrender to it. Start with some set of sensory stimuli that are simple, consistent, and frequently available, such as the alphabet, the numbers, or musical notes.
Pay attention to them, and notice any percepts (most likely colors) that might be associated with them. Initially, if you get anything at all, it will be weak and inconsistent. It's a bit like waiting for shy animals to come out to play. You can't rush it, you can't shape it, you can't even focus on it too directly. If you do, it will vanish like a dream. (Later, if you are successful, the percepts will become so strong and stable that you couldn't move them if you wanted to.)
The process is not quick or simple. It will be frustrating, and possibly disorienting. I would expect it to take at least several months, but I can't tell you for sure, because all I've found in the literature is a general presumption that it can't be done at all. But if those odds aren't too long for you, synesthesia can open up a new world.
Personally, having seen people who could not walk become able to walk, simply by altering their visual field, I am more inclined to believe things are possible than that they are not.
http://www.esnips.com/doc/e1d7c977-e25f-4494-8de0-16e00fb17e37/The-Treatment-of-Akinesia-Using-Virtual-Images
Here is a study showing that at least taste-smell synesthesia appears to be learnable.
http://www.nysaes.cornell.edu/fst/faculty/acree/fs616/pdf/Stevenson99.pdf
Finally, here is more on my own experience with synesthesia.
http://www.esnips.com/doc/31089b25-2f73-49d0-8ccc-07d9e938e7a7/Applied-Synesthesia-2005
