You can find great articles inside MSDN of Microsoft. One of those things is a list of ten tips improving yourself using SharePoint Server 2007 together with Excel Services. And here they are:
Making an Excel Workbook Compatible with Excel Services
Configuring External Data Connections
Configuring Excel Services to Work with Kerberos
Saving an Excel Workbook to a SharePoint Document Library Programmatically
Avoiding Exceptions in a UDF When Accessing the Caller's Identity
Handling Dimension Issues for UDFs That Return Arrays
Calling 32-Bit Native DLLs from UDFs on 64-Bit Operating Systems
Quick three Tips for Common Excel Services Issues
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