In the UK we are only just waking up to the joy and complexity that can be found in an espresso. To start at the beginning an espresso is a drink that is brewed from (finely) ground coffee beans using heated and pressurised water (ideally 95 degrees Celsius and 8-9 bars of pressure). A decent espresso is not a bitter caffeine-packed ordeal, it is a caramel-sweet sensation that tastes like freshly roasted coffee smells.
The reason that a properly brewed espresso is so wonderful is all to do with the way the pressurised water interacts with the tightly packed 'puck' in the filter head. The water extracts all of the aromatic oils in the coffee without any of the bitterness, due to the short extraction time (30 seconds maximum, 25 seconds ideally, all other factors being equal). Further, the pressure also cause these oils to form a rich microfoam known as 'crema' in Italian. One of the true marks of a good 'barista' (an expert espresso maker) is the ability to produce a 'Guiness effect' through the copious production of crema. The crema should last as long as there is any liquid left in the cup, it should definitely not be a thin scum that disperses after 30 seconds!
What is the proper technique for brewing espresso? Well first you need a decent machine (by which I mean you have to spend at least £275) that can produce the right amount of pressure. The best machine for the home enthusiast is the Rancilio Silvia, which I own. Just as important is a good burr grinder (not a blade grinder). I have a Gaggia MDF grinder which is adequate (£150). The next challenge in pulling a good shot is to find a source of good fresh roasted coffee beans. People are now familiar with the need to by whole beans to maximise their flavour, but grinding at home is not enough. Beans are only really good for a few weeks after roasting, and most vendors will happily keep beans in the shop for much longer than that. I'm currently using James' Gourmet Coffee who are not bad.
Once you have your machine, grinder, and beans you need to get the machine on and warming up while you 'dial in' the grinder, which means that you have to get the right grind setting for the beans. What is the right setting? Well to understand that you have to know about the rest of the process. A double shot requires 14-18g of ground coffee (most grinders will have a doser) and once this is in the portafilter you need to tamp it down with about 35 pounds of pressure. Again the exact pressure depends on other factors, but the whole process is improved by a decent tamper. Reg Barber Enterprises produce the best IMHO. Now you are ready to lock and load. Place the portafilter firmly onto the machine, turn on the pump, and watch the stream of coffee. After the initial application of pressure by the pump the golden liquid should drip from the portafilter with the consistency of warm honey. You should get about 2oz of espresso after roughly 25 seconds if things are right. If the coffee comes out too quickly you need to increase the tamp pressure or the grind (or both). Too slow and you need to do the opposite. With each set of beans this process of homing in on the right factors begins afresh.
If you are lucky you will be left with a few taplespoons of golden elixir, get it down your neck quickly while it is at its peak, but don't forget to knock out the portafilter and run some water through the head quickly to help clean the machine and then give it a wipe. The oils in coffee beans go rancid on exposure to air, and so any left on the machine will ruin your chances of reproducing your noble creation.