Sounds like a Colon Cleanse pitch no?
Well it's not. One of the most important things you can do when performance tuning is to remember to clear the procedure cache between test cases.
Don't be fooled by the word procedure. The procedure cache deals with almost all queries submitted to the SQL Server engine, not just those that live in stored procedures. It stores compiled execution plans for later use which helps speed things up by not having to regenerate execution plans when one is already available.
Back to clearing . . .
It's important to clear the cache between query submissions so that we start from scratch when we submit our new query. If we have "left-over" plans in the cache the optimizer won't necessarily create a newly optimized plan thus corrupting your tuning/testing efforts.
DBCC FREEPROCCACHE will clear all execution plans from the cache causing all subsequent SQL statements, stored procs or not, to be recompiled the next time they run.
It's also helpful to run DBCC DROPCLEANBUFFERS to clear the data buffers. This will ensure more accurate testing as all queries will have to retrieve their data from disk. Running CHECKPOINT prior to DROPCLEANBUFFERS will move all dirty data pages to disk, even further ensuring accurate testing results.