DBCC FREEPROCCACHE
Will remove all cached procedures execution plans. This would cause all subsequent procedure calls to be recompiled.
Adding WITH RECOMPILE to a procedure definition would cause the procedure to be recompiled every time it was called.
I do not believe that (in SQL 2005 or earlier) there is any way to clear the procedrue cache of a single procedures execution plan, and I’d doubt you could do it in 2008 either.