Athletes find it desirable to raise blood count to improve endurance performance. Specifically, they want a higher concentration of red blood cells (hematocrit) in their blood volume so more oxygen can be carried from the lungs to the muscles. A few options are available to increase hematocrit levels. The artificial options (medicine, transfusion) tend to be banned, but there are natural options as well. The natural ones entail stimulating the oxygen sensors in kidneys, which in turn stimulate the production of erythropoietin (EPO), which in turn stimulates more red blood cell production.
Live high but train low. Move to a high altitude, but train during the day at a lower altitude. Breathing the thin air between training sessions triggers increased production of red blood cells. Training at low altitude prevents the lungs from being exhausted before the muscles are fully exercised.
Sleep under a hypobaric tent. Hypobaric tents have lower oxygen levels than the atmosphere, thus triggering your body to respond with increased red blood cell production.
Breathe from a gas tank deficient in oxygen. Cyclist Eddy Merckx used this method to prepare for the altitude in Mexico City for his hour record in 1972. The way he used it qualified as "live low, train high," since he breathed from it when training on a stationary bike, the opposite of what is advised nowadays.