Q. How can I avoid changing the data source each time my source data grows?
A. Typically, when you create a Pivot Table from an Excel table, you click in the middle of it. Usually, after choosing Insert>Tables>Pivot Table, you just click OK. Next time, observe the Table/Range field in the Create Pivot Table dialog box. It should be set to the dimensions of the data source. Here, you can extend the end of the range by however many rows you think your data might grow to.
There are two alternative procedures. If your Pivot Table has already been created, click Change Data Source on the Pivot Table Tools Options contextual tab and extend the range here in the Change Pivot Table Data Source dialog box. A third option involves naming the range for your data source. Include as many rows as you think your data source will need over time. When you create your Pivot Table or change the data source, replace the range defined with the name of the range. Either method will avoid the problem.