While you work on your newest indexes / table design it is often necessary to clean up the cache on Sql Server to ensure that your changes are actually making a difference.
Typically to clear the cache you'll want to run, DBCC DROPCLEANBUFFERS.
If you are also executing your test via sprocs you'll want to clear that cache as well to do that just run, DBCC FREEPROCCACHE.
This ensures that you are always starting from the same point at each run. If you do not clear the cache between tests you can't be sure that your time results are accurate.