The Canadian Government's Guide to the United States

While a lot of us joke that what makes us Canadian is a near universal belief that "we're not American", this presumably unintentionally hilarious guide put out by the Canadian Foreign Affairs Department helps to guide us on how best to interact with Americans so as not to make them feel uncomfortable (via Carson):

In the US, much like in Canada, the personal bubble seems to range between 2 to 3 feet with closer allowances for louder places (such as a night-club or sporting event). In this context, you will usually get closer to allow yourself to be heard and then back away while reaffirming what you just said with the corresponding facial expression.

In a more intimate setting, the space bubble will decrease. In general, you don’t want to be "in someone’s face". 

Eye contact is good as long as it is not confrontational (don’t stare). It usually denotes interest and understanding. It is good to nod your head in agreement occasionally to show you are engaged in the conversation. 

As in Canada, people are not very tactile at first encounter. There is of course the customary handshake when you meet (right hand only, not too strong), and when you leave. In a social setting, when you don’t know people very well, a smile and wave will usually do. If you really hit it off with someone, you may graduate to the hug and kiss on both cheeks. This is not the norm in a business setting. 

In business, you will always want to a firm handshake; look the person in the eye while doing so. If someone shakes your hand without letting go, you can apply slight pressure with your other hand to their shaking arm’s elbow, holding the elbow lightly. This seems to work as a subtle cue to "let go".