You can save useful expressions for later reference. All persisted data is held in the so-called plugin-content-area in Eclipse which is found as a folder with the plugin-id as name under the path (Workspace)/.metadata/.plugins. The XPath-plugin generates one .xml-file, xPathExpressions.xml, which hold the persisted XPath-expressions.
To keep the current expression for later reference, simply press the button 'Keep XPath Exp.' in the cool-bar or select the entry from the drop-down menu (see above) with the expression entered into the corresponding field. This will add the current String to the list of expressions available in the XPath-expression combo. Once the plugin is deactivated by the workbench, this list is persisted into the file mentioned above.
To load a previous XPath-expression, simply select it in the expression combo list.
To delete saved expressions from the list, use the drop-down menu of the view (see above). This offers an action 'Organize XPath Exp.s...'.
A dialog will open, displaying the list of currently saved expressions. Select the expression(s) to delete and press the 'Delete'-button in the dialog. When you are done, press 'Close'.