Let's assume your espresso machine's brew pressure is properly regulated and the brew temperature is correct. I recommend setting the brew pressure to 8.2 to 9.5 bar and the brew temperature to 202 degrees Farenheit as a good starting point. Given that you've selected a quality grinder and fresh espresso blend, the final result now hinges on the extraction. And a lot happens in the twenty-five seconds of an espresso pour. If you have a standard portafilter, your observation is limited to the stream as it exits the spout. The bottomless portafilter offers a new vantage point to part of this process.
Before getting into the common extraction flaws, I'll briefly cover how to properly prepare the coffee for extraction, loosely based on the techniques proscribed by David Schomer, owner of Vivace and influential barista trainer, in his article Espresso Packing Techniques: Update 2004. The first page of the online article is a step-by-step reference card; the scanned text is hard to read, so I've excerpted portions of it below, starting with the third picture:
Using the steps above as guidelines, let's review dosing, distribution, and tamping in a little more detail.
Dose: A barista working in a commercial environment is making espresso after espresso. Practically all professional baristas dose the coffee grinds by volume. The techniques they use to assure the same amount of coffee for each espresso varies, but the net weight from shot-to-shot should ideally be one-half gram or less.
The production speed requirements for a typical home barista are less demanding and the dosing techniques can be adjusted to better conform to a low-volume pace. Some home baristas, who have the luxury of choosing only to cater to small groups, weigh out the coffee beans and run the grinder until it is empty, then sweep the doser clean. This eliminates coffee waste, but adds extra steps. Still, if you have an analytical bent and a precise scale handy, weighing helps assure consistency, especially when first learning. I often tare each measure of beans when I'm in "research mode" and switch to volume dosing once I'm comfortable with a particular espresso machine's use. It saves coffee and eliminates a source of shot-to-shot variance.
Whatever method you choose, keep in mind that the thickness of the puck is more important than the precise weight. If the puck is too high in the basket, locking into the grouphead will slice the top of the puck, especially on the right-hand side. This can lead to side channeling as shown in the first image of the Hall of Shame.
The stock double basket capacity of most espresso machines is approximately 14 grams. That is, if you cut a level over a gently filled basket the same way you would measure flour, it will contain nearly precisely 14 grams of coffee. Following the compression strokes prescribed above, the basket will contain closer to 16 grams of coffee, hence why it is called an "overdosed" basket. The optimal weight will vary for the grind setting, beans, and type of grinder (i.e., any of the grinders in the site's summary review will produce a "fluffier" grind than say a Rocky Rancilio).
It is important to have some clearance between the dispersion screen and the top of the puck. This facilitates the even distribution of water over the surface and allows the puck to expand upward to meet the dispersion screen as it absorbs water. At the maximum dose, a coin (2mm thick) placed on the top of the puck will graze the dispersion screen when the portafilter is tightened down, but recall that if the puck is getting grated on lock in, side channeling is likely.
Distribute: It's tempting to blame an uneven extraction on an inconsistent tamp, but in the majority of cases, improper distribution is at fault.
I prefer to overdose the basket slightly differently than the steps described by David Schomer. Instead of overdosing by compression strokes, I gently tap the portafilter on the grinder fork twice when the basket it about 3/4 full, and then finishing filling. Finally I do four leveling drags South-North, North-South, West-East, and East-West without compression.
The particulars of my preferred technique versus others you may read on the web are less important than consistency. Consistency is the key to continuous improvement and the ability to diagnose problems when they invariably occur. Inattention to small steps in your process can inadvertently add variability. For example, I saw during a recent barista competition some competitors who would tap the portafilter once to settle the grounds for their first shot and four times the next, followed by a finishing dose to the top of the basket. A finely ground coffee powder will settle slightly with each tap, so these competitors were likely seeing one or two gram variances between shots, which adds or removes several seconds to the pour time.
Tamp: The purpose of tamping is to improve the density consistency of the coffee puck. Left untamped, the high pressure of the extraction is more likely to open fissures in the puck. As defined earlier, the increased flow of water through these fissures is known as channeling, and results in water that would otherwise go to extracting coffee solids evenly throughout the puck being concentrated along a narrow pathway. Flavor and body characteristics of such an espresso are very similar to the final seconds of an extraction after the onset of blonding: Thin, nearly flavorless, and no sweetness.
Hold the tamper handle like you would grasp a doorknob, that is, with the shaft along the length of your hand and the end resting in your palm. Applying thirty to forty pounds of tamping pressure is often quoted as the standard for professional baristas, partially because of concerns about occupational overuse. This is also a good guideline for the home barista, but again, consistency should be your primary concern. Consider using a training tamper like the Espro, which is calibrated to thirty pounds, or using an inexpensive bathroom scale to train yourself how to apply correct and consistent pressure.
My preference is the so-called "Staub" tamp, credited to Carl Staub. The Staub tamp focuses on getting the coffee off the sides of the basket by tamping once in the center, four times at the points of the compass, each time lifting the tamper out of the basket, and finally a light polishing tamp. There's not a lot of clearance between a tamper and the sides of the basket, so saying that you're tamping the "edge" is a bit of a misnomer. Professionals may not have the time for the added steps with a long line of harried customers, but since I have only a few tries at a great shot during the work-week before heading out the door, I believe it is worth the extra time assuring that the tamp is not canted and the puck is tamped firmly all along its perimeter.