Start with a lip liner in a nude color, and lightly draw a line just slightly outside your natural lip line. Smudge the liner for a more natural look. Avoid dark liner and don’t draw too far outside your natural lip line.
Fill in your lips with a pale shade like MAC High Tea or Laura Mercier Passionfruit. These both have a bit of frost, which will reflect light and bring out the fullness in your lips. If you don’t care for frost, choose a color that's a shade lighter than what you usually wear. When you highlight, you bring forward, and when you contour, you sink in, so light colors on lips will appear to plump them up, while dark colors tend to make them recede and look smaller.
Finish with a bit of gloss in the center of the top and bottom lips, where your lips are naturally the fullest. Look for a gloss with a bit of sparkle, to reflect the most light, like Prestige Wonder-Full Lip Plumping Gloss in Deluxe. It’s a bit minty and doesn’t slide around.
If you’re happy with your lip color but want subtly fuller lips, then simply put a dab of lighter gloss in the center of your upper and lower lips to create the illusion of fuller lips.