The essential of the economy relates to human to human exchange. Whether this is through information or the use of a monetary system it is important to understand that economies are connected through interaction. Considering that the study of economics is actually the study of human behavior on a macro level one should come to the conclusion that the borders of an economy are where interactions are less.
What makes an economy a single entity?
It is the speed and volume of interaction that occurs among people. For example, a state is defined as an entity because the people within it share certain behaviors and patterns that lead to an understanding of shared economic interest and governance.
How does one improve the economy?
By encouraging greater interaction of the fundamental elements-humans. As they share information and exchange monetary value they increase the speed of transactions. More transactions typically also means additional revenue generation which leads to additional taxes on each of the transactions report as income.