OK. If you were training to climb at altitude, you'd need to spend a lot of time up there, a week or so for every 1000 meters, because there are many phsiological changes which occur to adapt to high altitude. However if you are just trying to improve your sea level performance, what you are after is increased hematocrit. You get that from the release (or injection) of erythropoetin (EPO).

The kidneys release EPO when they sense below normal blood oxygen saturation. There are several ways to create this lower saturation. Sleeping at high altitude or in a hyperbaric tent is one. Hypoxic breathing is another. However the most common and most useful is simply anaerobic intervals. They not only create the hypoxic state, they make you stronger, both mentally and physically. So that's what most cyclists do.

A set of 3X3X3 intervals well above lactate threshold once or twice a week will do the trick. In 2 weeks to a month, your body will respond to the stimulus by creating more red blood cells, thus increasing your hematocrit and your performance.

You should have a good bit of base and LT or sub-Lt interval work before doing these harder intervals.