If you for example need to test a query against two SQL servers hardware, its a good idea to clear the cache first.
This query can do this:
DBCC DROPCLEANBUFFERS;
-- Removes all elements from the plan cache.
DBCC FREEPROCCACHE;
-- Displays the number of milliseconds required to parse, compile, and execute each statement.
SET STATISTICS TIME ON;
-- Display information regarding the amount of disk activity generated by Transact-SQL statements.
SET STATISTICS IO ON;