From Diane Ackerman's essay on "Kissing" in A Natural History of Love:
"According to the anthropologists, the lips remind us of the labia, because they flush red and swell when aroused, which is the conscious or subconscious reason women have always made them look even redder with lipstick."
"Kissing, we share on breath, open the sealed fortress of our body to our lover. We shelter under a warm net of kisses....Setting out on a caravan of the other's body, we map the new terrain with our fingertips and lips, pausing at the oasis of a nipple, the hillock of a thigh, the backbone meandering riverbed. It is a kind of pilgrimage of touch, which leads to the temple of our desire...."
"So, anthropologically at least, a kiss on the mouth, especially with all the plunging of tongues and the exchanging of saliva, is another form of intercourse. No surprise that it makes the mind and body surge with gorgeous sensations."
Given the similitude between a woman's upper and lower lips, it's fun to play with this likeness in a kiss (or a sequence of kisses).
Here's my suggestion for a type of sensual kissing: a labial kiss of the upper lips.
Let the man pretend that the upper lips are the labial lips, and let him kiss these lips slowly, very slowly, as if his tongue and kisses are making the lower lips of a woman petal-open and honeyed-up.
The tongue, in this labial kiss, can be very teasing, just touching and tantalizing one lip at a time, sliding with grace and a bit of poignant pressure, back and forth along the upper lips, before descending to do the same for the lower lip....and before sliding the tongue deeper into the mouth.
It can be most fun if the woman pretends to be a bit coy, as if defending the castle or inner sanctum of her most womanly self, with something of a moat: the lips can be very wet and moist, but the drawbridge remains up, resisting entry...for a time.
The labial kissing can be very mind-blowing or mind altering: The upper lips are horizontal, not vertical, but if the analogy is felt by both partners, heads can be adjusted by 90 degrees, so the upper lips can be approached vertically as well, in closer simulacrum to cunnilingus.
The erotic imagination can be so powerful: The labial lip kiss can become even more exciting and playful if the man imagines that in the corner of the lips, right where top and bottom lips merge together, there is located an imaginary clitoris that can be taken and teased by the tip of the tongue.
I daresay that 15-20 minutes of beautiful kissing in this labial style...with a man taking time to part open the upper lips and then to tango slowly and beautifully inside with the woman's tongue once entrance has been secured will shorten the time necessary to part open the lower lips of a woman.
Indeed, when the upper lips have been taken beautifully, the lower ones will be begging as much for the touch that opens them up as a garden flower desires the morning rays of sun.
The above description is for the man on top position and the woman the bottom, but it can be very hot--and instructive--for a switch to occur: Let the woman take the man's lips and kiss his lips in a way that reveals how she loves to be taken by the tongue down below.
Yes, women, if you've ever wanted to gently instruct a man how to pleasure you down below, just take his upper lips by finger and tongue the same way you love to be taken down below, doing unto his upper lips what you love done unto your lower lips.
And don't be shy about using a finger as an instructional pointer, parting open and entering and then trusting into his mouth as you love to be taken by finger and cock down below