To create illusion of a larger upper lip, use lip liner and draw the line above the top lip line.
To create illusion of a larger lower lip, use lip liner and draw the line slightly below the bottom lip line.
Try to apply lipstick starting from the outer corner of the mouth, going up toward the bow at the centre of your top lip, you will create effect of fuller lips with softer lines. The other benefit is, you will always have perfectly colored lips, without missing the outer portion of your lips.
Use lipsticks in light, glossy, shimmery shades because they will make your lips look fuller. Avoid dark, matte shades because they make lips look thinner.
If nude lips are more in your comfort zone, choose a shade that slightly contrasts your skin tone - otherwise it seems like lips disappear. For fair skin, that means a pink based nude; for medium a toffee-coloured hue; and for dark skin, a caramel-coloured nude. Nude lips look the most natural without tons of sparkle, so skip the shimmery varieties.
When you want to make your lips look larger, don't go far from their natural edges, because trick can look too obvious and it will not work. Remember, you want to enhance your lips, not create new ones!
If you dab a bit of lip gloss over the lipstick, you will add dimension and make your lips look fuller.