Here is a short one on the go.
If you work with reports on your EPM data and you want to use Excel Services to share the report with your colleagues you should display the data in a pivot table or pivot charts.
Because external data can only be refreshed in the browser, if it is displayed in a pivot table! Do not use external data ranges! They can only be refreshed using the Excel Desktop Application. With pivot tables / charts you are always up to date and good to go!