Obviously it's not only physical. Some major factors before even getting in to it:
1. Time
Time of month, time of day, time of year. If it is the wrong time, it ain't going to happen.
2. Mood
She better be very happy and ready to do anything. She needs to be confident and ready to take the next step with you.
3. Environment
From music to your room setting. If you're a slop and ill-prepared, nothing is going to happen.
You may think those are basic, but so many "clueless" men get into it without ever thinking about those four basic things. I know sad, very sad.
Step 1: Making the Right MoveIf you meet the other criteria I listed, and that is not easy to do, your shot at a one-night stand or successfully making love will improve. Regardless of the occasion this strategy will work on most women (80%). The exception would be super models, I am just guessing here. Before I get into this first stop of foreplay (sorry guys there are 2 steps), let me credit Men's Health for really being a help. That magazine has been the best investment I as a guy have ever made.
You must understand guys, women will not get naked if they feel insecure (ie: they have tons of flaws). If you want them naked (especially in the light), give them confidence. This is done by romantically complimenting them. Notice the romantic? That does not mean saying saying, "Your legs are as smooth as my condom." Wrong, but I know plenty of guys that would pull that line. although I would not compliment her legs at first (start with the face), if you are going to shoot first with her legs say, "Your legs are as soft as a cool breeze." That sounds ten times more romantic and sweet. Women's insecurity is majorly in three areas (according to Cosmo):
1. Butt
2. Legs/Hips
3. Boobs
Ironically, though, women think it is most romantic if you first compliment them on their eyes and hair. Like I stated start with facial features. You can move your way down after that, but remember that the lower you get too early, the less chance of anything happening. Stay focused on her facial area as you compliment her and look her in her eyes with wonder. She'll realize that this is more than just some good fun, but something that is going to be special. Men tend to forget a lot of times that it is about an intimate connection during sex.
Guys also don't get speech sometimes. Females prefer the gentle whisper over rough yelling (we're talking about at the beginning). I dated a girl who loved rough dirty talk, and I ignored that part about her and pulled off my gentle whispers trick. She loves whispering now even more. Practice makes perfect, so if you're not quite sure how you sound get a tape recorder and find out and just practice. But you have to be good ...
To do it right: a soft whisper very lightly blowing near where her ear is (note: not necessarily in her ear, but near it). Compliment her romantically very gently, make her want to kiss you BAD.
Whatever ear you are close to (right or left) go to the nearest cheek and gently kiss and blow very softly, move a half an inch towards her lips, kiss and blow again. Cross over her lips not kissing them but kissing and blowing her other cheek until you get to her ear, and then romantically compliment her some more. Repeat this several times, make her beg to kiss you, but don't give in yet. She just might beat you to it though ... Also after a few times around it is ok to do a LITTLE (notice that word there) wetter kisses, as these can really drive a girl nuts when you blow on them. Some guys have used a special kind of peppermint that give a tingly sensation when you blow on it. Basically you get the girls cheek a little wet and then blow. Yeah, it drives a girls nut, but this is just the beginning!
So finally your lips meet. Go gently at first, men like it rough too quickly. All women have different tastes when it comes to making out, but here are a few women tend to HATE:
1. The hanging tongue: letting your tongue hang in her mouth without movement.
2. The darting tongue: no explanation needed.
3. Playing with teeth equipment: again no explanation needed.
Some they tend to LIKE:
1. Gently rubbing your tongue against their top lip causing a tingling sensation (the peppermint can REALLY enhance this!).
2. Gently playing tongue tennis.
3. Gently stroking inside lip or top of mouth with tongue.
Amazing how it's all soft and gentle! Since every woman is different pay attention to what she likes, you'll notice this by rapid breathing and moaning - this right here is the greatest indication that what you are doing is working. The same strategy goes for kissing her neck, gently soft blowing and kissing. You'll see what I mean. Patience will lead to some awesome sex before it's all over with, you'll see.
Before I go on, let me just say if your girlfriend does not want to go any farther than my last step mentioned respect her!!! It is not ever a gentleman like thing to push a girl to do something she is not ready to do. If she doesn't want to have sex until marriage, respect her!
That being said, if she does want to go further, and you followed the last steps you will know right off. She will begin to want you to do things "farther down." It would be much easier if she got fully naked for this next step, but if she only takes off her shirt and bra, go with that.
This next step required one of three or all of three things:
1. Tongue
2. Massage oil/hands
3. Feather
A diamond may be a girl's best friend in life, but a feather is a girl's best friend in bed - trust me!

Depending on how fully naked she is you are going to gently stroke her with feather/oiled hand/tongue all over her body except three areas: her two nipples, and her vagina/clitoris area. You will gently stroke every place on her body going around those areas very gently for quite some time. Each time you go over her body once, then go and whisper something a little naughty and very gently in her ear. Saying something like "Your goosebumps are incredibly sexy."
Top places girls loved to rubbed in:
1. Behind the knees
2. Neck
3. Back
4. Legs/hips
5. Feet
6. Around vagina area
7. Around boobs
Basically their whole body. Spend a great deal of time doing this. The girl will very quickly beg you to "go inside of her." That is because she will get incredibly turned on if you do this right.
After much begging, don't go inside of her, instead rub the feather/oiled hands/tongue over her nipples. Now gently stroke her entire body including her nipples, and gently go around her vagina area. Blow softly when you go over this area.
(BONUS TIP: To add some pleasure and fun take a couple of mints and kiss her really wetly with "minty breath" then blow gently on the wet spot. This causes a tingling sensation if done right.)
Very soon she will be begging, maybe even forcing you to go inside of her. Basically, she's primed and ready. But guess what? You still aren't going to get inside. First you are going to do some oral action with her clit (Next step). Before you get there make sure to whisper more dirty thoughts in her ear and kiss her cheeks and lips. Also kiss her eyes (when they are closed). Tell her she is beautiful and tell her what you are about to do (in a romantic, and gently dirty way).
Once again be sure to be gentle and patient. By this time 99% of men are so hard that their patience is growing thin, but trust me your pay off will be so worth it.