People who like to touch really like touching, and people who do not like to touch really dislike being touched. You will need to watch your colleagues for clues on what they are comfortable with.
Americans are generally uncomfortable with same-sex touching, especially between males.
Holding the middle finger up by itself is considered insulting and vulgar.
Americans smile a great deal, even at strangers. They like to have their smiles returned.
Men and women will sit with legs crossed at the ankles or knees, or one ankle crossed on the knee.
Some Americans are known as "back slappers" -- they give others a light slap on the back to show friendship.