The basic flourish shape is a figure eight, ribbon or infinity loop. Practice this until the motion becomes smooth and natural. Notice how the sets of loops run parallel to each other and the width of each level can expand and contract. Work with plenty of arm room and move your entire arm through the flourish instead of just your wrist to get the most natural line shapes.
Flourishes should be a natural extension of the letter they extend from, so use them with the more flowing hands where serifs are a natural jumping-off point for the flourish as opposed to more angular types.
Use two pencils held together (creating a double point) to duplicate the thick and thin nature of a furling ribbon. Ink over these lines in one color and fill in with another for a two-tone effect or leave the bare pencil for a simpler look.
When using a metal calligraphy nib, press gently on the upstrokes to avoid gouging your paper or splattering ink. Apply firmer pressure on the down strokes to take advantage of gravity and varying line widths.
Flourishes can surround a single letter to create the look of a monogram when alone on a page or to suggest a more illuminated look at the beginning of a paragraph or document.
Individual lines can be used in a flourish--the art is not limited to continuous figures. Explore extending the crosses of t's, h's or other letters to act as base lines to flourish around.