Well, many top caliber athletes have been known to do blood doping. Basically this amount to training at higher altitudes to cause the body to generate more blood cells to compensate for the thinner air at altitude. They then have blood drawn at certain intervals and stored for eventual infusion before major competition. IT is considered illegal if I am not mistaken - you can read more about it at
http://whyfiles.org/090doping_sport/3.html
You can also do a search via the major search engines under the phrase "Blood Doping"
Hope this helps.
