From afar, the first task of body language is to signal interest (and then to watch for reciprocal body language).
The eyes do much signaling. Initially and from a distance, a person may look at you for slightly longer than normal, then look away, then look back up at you, again for a longer period.
There are many preening gestures. What you are basically saying with this is 'I am making myself look good for you'. This includes tossing of the head, brushing hair with hand, polishing spectacles and brushing clothes.
Remote romantic language may also include enactment of sexually stimulating activities, for example caressing oneself, for example stroking arms, leg or face. This may either say 'I would like to stroke you like this' or 'I would like you to stroke me like this'.
Similarly, the person (women in particular) may lick and purse their lips into a kiss shape and leave their mouth slightly open in imitation of sexual readiness.
Objects held may be also used in enactment displays, including cigarettes and wine glasses, for example rolling and stroking them.
Attractive parts of the body may be exposed, thrust forward, wiggled or otherwise highlighted. For women this includes breasts, neck, bottom and legs. For men it includes a muscular torso, arms or legs, and particularly the crotch (note that women seldom do this).
Faking often happens. Pressing together muscles gives the impression of higher muscle tone. Pressing together and lifting breasts (sometimes helped with an appropriate brassiere) makes them look firmer and larger. Holding out shoulders and arms makes the body look bigger. Holding in the abdomen gives the impression of a firm tummy.
This is often playing to primitive needs. Women show that they are healthy and that they are able to bear and feed the man's child. The man shows he is virile, strong and able to protect the woman and her child.
Leaning your body towards another person says 'I would like to be closer to you'. It also tests to see whether they lean towards you or away from you. It can start with the head with a simple tilt or may use the entire torso. This may be coupled with listening intently to what they say, again showing particular interest in them.
A person who is interested in you may subtly point at you with a foot, knee, arm or head. It is effectively a signal that says 'I would like to go in this direction'.
Other forms of more distant display that are intended to attract include:
Sensual or dramatic dancing (too dramatic, and it can have the opposite effect). Crotch display, where (particularly male) legs are held apart to show off genitalia. Faked interest in others, to invoke envy or hurry a closer engagement. Nodding gently, as if to say 'Yes, I do like you.'
When you are close to the other person, the body language progressively gets more intimate until one person signals 'enough'.
Close in and personal
In moving closer to the other person, you move from social space into their personal body space, showing how you would like to get even closer to them, perhaps holding them and more...
Standing square-on to them also blocks anyone else from joining the conversation and signals to others to stay away.
Imitating the person in some way shows 'I am like you'. This can range from a similar body position to using the same gestures and language.
When you are standing close to them, you will holding each other's gaze for longer and longer periods before looking away. You many also use what are called 'doe eyes' or 'bedroom eyes', which are often slightly moist and with the head inclined slightly down.
Where the eyes go is important. Looking at lips means 'I want to kiss'. Looking at other parts of the body may mean 'I want to touch'.
A very subtle signal that few realize is that the eyes will dilate such that the dark pupils get much bigger (this is one reason why dark-eyed people can seem attractive).
Touching signals even closer intimacy. It may start with 'accidental' brushing, followed by touching of 'safe' parts of the body such as arms or back.
Caressing is gentle stroking that may start in the safer regions and then stray (especially when alone) to sexual regions.