Since this act is so old, no one is really sure where it came from. There are a few generally accepted ideas, the main one being that it was used to indicate to the other person that you were not holding any weapons. The shaking was to jar loose any hidden weapons. Many agreements were held to based on just a handshake, and it was considered as important as any legal documents you'd sign today. Here is more information how how to give a good handshake.
It's contact with someone, normally considered peaceful and non-sexual.
Grip firmly but not enough to shatter the bones. Careful when shaking the hand of an elderly person, as their hands may be sensitive. In this situation, grip only as firmly as they do.
Shake firmly. Limp handshakes are Hollywood movies; they leave you feeling dissatisfied. If you're one of those people who just can not stand to have other people touch their skin for fear of germs, invest in some thin latex gloves. Do not use your fear of germs to give a limp handshake.
Remember to let go. Sometimes it's easy to get carried away and not want to let go. If you hang on too long, let's face it, you're being annoying. Family members are guilty of this at times, and drunken elderly men.
Don't grip too hard. It sucks to go to shake someone's hand only to have it crushed. People who are wearing rings should be careful when shaking hands, as it hurts a lot when your fingers get squeezed together by someone who grips too hard.
Pumping someone's hand up and down may look funny but it's not. Kids seem to enjoy doing this, and as adults, we have no reason to do this sort of thing to each other.
Sweaty palms. If you even think that you'll be shaking someone's hand, wipe it on your pants or something. A wet handshake is a nasty handshake.
Clean hands. If your hands are not clean, do not shake anyone's hand unless their hand is equally dirty. On a construction site, for instance, is a good example of when both people will have dirty hands, yet still want to shake.
Don't lean into them, or put their hand dangerously close to your groin.
Don't be too far away, or step back as you're hanging on.
Shake only one of their hands. Grabbing both just makes you look stupid and they may laugh but they're laughing because you're an idiot.
Always look the person in the eye. Not only does it let them know you are giving them a proper greeting, but if they are saying their name, you may be more able to remember it if looking at their face as they say it.
If in a group, shake as you are introduced, or begin to your right and continue counter clockwise. If you are introducing yourself, if you are to speak first, you will want to say something such as "Hello. I'm Eiddon." Hopefully, they will tell you their first name, and then you shake hands. Some people put their hand out at the "Hello", shaking as they say their name, and then release once the other person has said theirs. Always state your name clearly, and the way you want to be addressed, if your name has variations. (If your name is Edward, but you go by Ed, then introduce yourself as Ed.)
Use your right hand. If the person you go to shake the hand of doesn't have a right hand, then use your left and shake their left. If they have no hands, then give a polite head bow (not a nod).
After shaking anyone's hand, do NOT touch your eyes, food or put your fingers in your mouth. That person could have been itching their ass and you do NOT want that in you. Wash your hands as soon as you can without become a neurotic freak. If you are going through a bar of soap a day, using scalding hot water and end up spending hours washing your hands, then you might have a problem and should go see your doctor immediately.
If two people are already shaking hands, do not reach prematurely towards either of them, or grab and shake their shaking hands. If you reach prematurely, you'll be standing there with your hand outstretched looking like a complete loser who can't manage to take visual cues from others. (Blind people can not be accused of this, as they can not see others reaching towards them.) Grabbing their shaking hands lets everyone around know that you're a needy loser who just can't wait for your turn to shake hands. Only morons do this.
Used as: Saying thank you for a job well done, introducing yourself to someone, greeting someone in a business or formal environment, a social greeting between friends and/or family, a gesture of agreement, a symbol for teamwork and/or fellowship.
Sometimes accompanied by a brief one to two second one-armed hug of the shoulder, and/or a kiss on the cheek. When going to a different country other than your own, be sure to look up these sorts of customs so you don't rudely jerk back when someone goes to kiss your cheeks.
During sporting event finals, the athletes will go give what is called 'good game' handshakes. This sees the two opposing teams line up and proceed to go through the line, shaking all of the opposing athlete's hands and saying "good game." This handshake is supposed to convey a sense of respect to the other team for playing well and indicate there are no hard feelings. Of course, this is rarely true, and most do not even look each other in the eye while shaking hands, conveying their lack of respect without openly saying it. These 'good game' handshakes have been reduced to a mere formality at even the pee-wee level of sporting when children are paraded out to shake hands and never told how to do so properly. This 'good game' handshake can also be extended into a 'good game' pat on the ass.
Sometimes handshakes are used when people do not feel comfortable enough to hug. To many, a firm handshake relays all their emotions if they are not the affectionate type. A firmly grasped hand can lend silent support, and relay many emotions.
Groups such as secret societies have been known to use handshakes in which only members are privy to. This is then used to gain entrance or prove one's alignment with said group. The reason this may be popular for these societies is that the handshake is a silent and quick action. Watching two members shake hands may not reveal the secret, even if observed by others. Some of these handshakes alter where you place your fingers, or involve moves after the initial shake of greeting.
Women & Handshakes - Many people are confused as to how to shake a woman's hand. In business, always shake men and women's hands in the same professional manner. If in an intimate setting, some prefer to accompany the handshake with a kiss to the hand. There are many women who don't appreciate strangers kissing them, so be careful when doing this or you may get hit in the face.
Children & Handshakes - Beware of dirty hands! Do not grip hard, as children have small fragile hands. Since their hands will not fit properly in adults hands, a modified grip is needed.
Pets & Handshakes - Some pets are able to 'shake a paw' with humans. As always, don't squeeze too hard, allow the dog to decide when the handshake is over.
Business & Handshakes - Still very important in many businesses. Even job interviews demand a handshake at the beginning and end.
Doing rude things (such as jamming your hand into your ass sweat) and then tricking people into shaking your hand. This has been shown on TV shows with hidden cameras, and even shown in the movie Mallrats. Sure, it was funny and well deserved in Mallrats, but innocent people do not deserve this sort of thing.
Do not try shaking hands with fish. They don't have any hands and shaking their fins/tails off doesn't count.
Farbars from the planet Far do not like being touched. Never attempt to shake them in any way.
Shaking hands is a good way to see if the person you are speaking with is actually real or not. Imaginary friends, ghosts and hallucinations are normally unable to give handshakes.