Microsoft® SQL Server 2012 Unleashed

Summary

One of the most important aspects of improving SQL Server performance is proper table and index design. Choosing the appropriate indexes for SQL Server to use to process queries involves thoroughly understanding the queries and transactions being run against the database, understanding the bias of the data, understanding how SQL Server uses indexes, and staying aware of the performance implications of over indexing tables in an OLTP environment. In general, you should consider using clustered indexes to support range retrievals or when data needs to be sorted in clustered index order; you should use nonclustered indexes for single- or discrete-row retrievals or when you can take advantage of index covering.

To really make good index ...