If you are using OpenQuery or sp_execute_sql a lot then your server is probably loaded with cached execution plans, for adhoc queries that will never be used again.
These two grate blogs deal with the issue nicely
http://sqlmag.com/database-performance-tuning/sql-server-plan-cache-junk-drawer-your-queries
http://www.sqlskills.com/blogs/kimberly/plan-cache-and-optimizing-for-adhoc-workloads/
