There are several ways. I would put a curve on an adjustment layer above the face (or lips, if they are on a separate layer, and above any other color adjustment layers.) Set it for luminosity mode. Now pull the master curve toward the light end of the spectrum until the lips are dramatically lighter—not blown out, but thoroughly washed out. Fill the curve's layer mask with black.
Now use a brush, size based on the distribution you wish of sparkles. Set the mode of the brush to dissolve, opacity to about 10 - 30, and if you're using a pen and tablet, go to the brush window and click Other Dynamics on, and set the Opacity Control to Pen Pressure.
Set your foreground color to white.
Now, paint into your mask wherever you desire sparkles. Dissolve mode scatters pixels randomly. Opacity settings determine how many random pixels are applied with each stroke, but they all come out at 100%, which, in this case, means that random white pixels are allowing your curve to affect the lip color, producing a nice sparkle. (Obviously, if a specific color effect is needed for the sparkles, like gold, then instead of simply wasing out the lips when you first set your curve, adjust your color curves until the lips are gold. Then your pixles will scatter gold sparkles.