What is wok hai and how do I get it in my food?
Wok Hei is the "essense" imparted by the wok when stir-frying over very high heat.  The characteristic flavor is due to the the Maillard reactions and the charring of the ingredients.

Practically speaking, most home stoves do not have sufficient BTU output to achieve Wok Hei. If you have a gas stove with sufficient output, consider yourself fortunate.  Get the wok really hot before adding the oil and ingredients... and stir vigorously.  You should also make sure to crank up your hood fan/ventilation, since there will be quite a bit of smoke generated.

For those without a high BTU stove (myself included), a suggestion was made 
here regarding a burner that could be hooked up to a propane tank.