To sear a piece of meat, heat a heavy bottomed skillet over medium-high heat until drops of water added to the skillet bounce around and immediately evaporate. Add a little bit of oil and swirl to coat the skillet evenly. Add the meat (you can season it with a bit of salt and pepper first)-it should start to sizzle immediately-if not, your pan is not hot enough. Leave the meat in the pan for 3-4 minutes, sizzling away, until its stickiness releases and you can easily turn it over with a pair of tongs. When you turn it over, you should have a nice, golden brown crust on the meat; if not, leave it there a little longer until you do. Turn it over and do the repeat the process with the other side of the meat. The goal is to achieve a nice dark, golden brown crust without burning the meat. This is called the Maillard reaction- or the constant shifting of sugars and proteins in the meat in reaction to heat. It's where the inherent flavor of the meat is intensified.
At this point you can move it to the oven to finish cooking, or make a pan sauce with some liquid and other seasonings.
Shirley Corriher in her book Cookwise notes that acid inhibits the Maillard reaction. Thus, you will not achieve the nice browning from the sear if you marinate the meat in a mixture that contains a low ph. Best to add any acid later in the flavoring process.