Athletes "used to take" EPO to increase hematocrit. That's what it does. It increases the number of red blood cells in the blood. The reason EPO usage was once hard to detect is because it is a natural chemical secreted by the kidneys. The stimulus to secrete EPO is oxygen debt or hypoxia. Which you can supply by sleeping in a low-oxygen environment like a hypoxic tent or high altitude, or doing anaerobic intervals. Here's a link:
http://www.dailypeloton.com/epo.asp
As you can read, it's a lot more complicated than just riding hard.

The relationship between intervals and hematocrit is complicated by the fact that intervals also increase blood volume, which decreases hematocrit, which is the concentration, not the number of red blood cells. Dehydration increases hematocrit. Be that as it may, it could be that it's the number of red blood cells, not the hematocrit, which is the best goal. And be that as it may, some pro teams have their riders sauna after a training session to increase hematocrit through dehydration. Which I don't get at all.