The plot is determined by the data points computed. You can increase the quality of the graph by increasing the Sample Size. Increasing the Sample Size also slows down the plotting process, of course. This is especially critical for Plot 3D because the number of data points computed is the produce of the two numbers entered under Sample Size. Thus the default number of data points computed is 252 = 625. So you may want to specify a relatively small sample size for real-time demonstrations and a relatively large sample size for printed documents.