Watch the Using Echoes online guitar lesson by Dave Celentano from Classical Concepts for Rock Guitar
Start by setting the delay to a single repeat (not a series of repeats that fade away) and make the volume of the repeated note equal to the actual note, so that the listener can't hear the difference between the actual and delayed note. The tempo of my delay to perform "Invention in D Minor" is approximately 132 b.p.m. Experiment with a slightly slower and faster echo tempo until you find the right spot. This technique is a bit trickier to master and involves doing the hammer-on from nowhere and volume swells in time with a steady beat so the single repeat dotted eighth note delay of the first note will echo between the second and third actual notes, and the second actual note echoes between the third and fourth notes, third actual note echoes between the fourth and fifth notes and so on.