Understanding sweet spotting:
Hammer poppet markers use some manner or ram to hit the stem of a radially sealing valve to fire the paintball. Markers such as autocockers, spyders, egos, and pretty much every stacked tube poppet use this system. http://www.zdspb.com/tech/misc/anim_hammer.html
This system relies on a balance between the force pushing the valve closed, and the force pushing the valve open to regulate air flow. The force pushing the valve open only exists at the front of the firing cycle and comes from the ram or hammer. The force holding the valve closed comes primarily from the valve spring, but the pressure it retains also has some bearing.
Generally as the pressure increases, so does the output velocity, but there will come a point when there is sufficient extra force from the pressure that increasing the pressure will actually being to restrict valve movement, and increases in pressure will actually cause a decrease in velocity. The pressure right on the edge of those two zones is the "sweet spot".
At the sweet spot, both a rise an a fall in pressure will result in a decrease in velocity, so at this point the marker will have the best consistency. Note however, that the location of the sweet spot can change based on the force of the hammer/ram. Finding the forward force that produces the appropriate sweet spot is the subject of this article.
Note that while the sweet spot may provide the best consistency, it is arguable if it is the best configuration for a marker, and that argument varies by marker.
Step 1: The method.
The first step is to find how to adjust your markers hammer force. For electropnumatic markers such as Egos and Intimidators, it will be controlled by dwell. For most mechanical markers such as Autocockers, Spyder clones, and Tippmanns, it will be controlled by the spring tension of the hammer. The method of adjusting this spring varies by marker. Note that many Spyder clones and Tippmanns do not come stock with regulators, and so unless retro fitted with them, cannot be sweet spotted.
Next, turn the hammer force (hereafter referred to as "dwell", because it is functionally equivalent) down dramatically. You should see a dramatic drop in velocity. Now turn the pressure up until the velocity will go no higher. If the velocity begins to drop, you've gone to far. This process essentially finds the peak of the curve illustrated previously. Note that it is a fuzzy region.
If that maximum velocity is the one you want, then congratulations! You got incredible lucky. In all likely hood, your velocity will be much lower than the one you desire. Raise the dwell in small increments(about 0.5 ms for electros, 1/4 turn for mechs), checking the maximum velocity each time as discussed above. Once the velocity peaks at whichever velocity you desire, you're done.
When checking velocity, you must take the average of at least 5 or 6 shots, or you may be mislead be variance in the markers output.