Be prepared to talk about your work in an intelligent way. Be able to note your major influences, sources of your imagery, and discuss your particular medium. Have a clear understanding of where you fit into the current contemporary art market and the role of your work in an art historical context. If you are walking into a gallery for an appointment, stop for a moment to observe the exhibition. After all, if you want them to be interested in your work, you should express an interest in theirs.
Gallery dealers invest more than time and money in the artists they represent.
Show an appreciation of their efforts. Be aware of the personal space around you. Don't walk in carrying an unwieldy load of 30 paintings. Arrive carrying only what you can comfortably carry in one hand, preferably your left, to make your right hand available for shaking. Greet the person with a handshake, offer them a resume or slides to look at, while you go back out to the car to fetch the rest of your work.