I would review the below from the SQL Server Customer Advisory Team. The article addresses the issues below:

(1) How can I find out whether my indexes are useful? How are they used?

(2) Do I have any tables or indexes that are not used (or rarely)

(3) What is the cost of index maintenance vs. its benefit?

(4) Do I have hot spots & index contention?

(5) Could I benefit from more (or less) indexes?

http://blogs.msdn.com/b/sqlcat/archive/2006/02/13/531339.aspx