I once had a surfing dream that was so real and so immersive that when I woke up I was gasping for air in a complete panic not quite sure how I ended up in my bedroom instead of trapped underwater by the currents at Zuma beach. (Sleep apnea?)
The dream saw me wiping out on a big wave and apparently I had held my breath in my sleep. Suddenly I woke up hyperventilating to get air in a full blown panic complete with a racing heart and adrenaline surge. Stuff like this happens to me when I visualize.
If I visualize a race, I will lose two seconds before stepping on the blocks because I will be hyperventilating by the time the bell goes off. Many champions like Greg Louganis et al. have had very good results through visualization. Here is an About.Com article about it: [Link]
Photo from youngdoo's photostream at Flickr.com.