How can I select the first day of a month in SQL

Question

I only need to choose the given DateTime variable's first day of the given month.

I am aware that utilizing this type of code makes it relatively simple to do:

select CAST(CAST(YEAR(@mydate) AS VARCHAR(4)) 
+ '/' + CAST(MONTH(@mydate) AS VARCHAR(2)) + '/01' AS DATETIME)

However, this is regrettably not very elegant and neither is it very quick.

Is there a more effective approach to this? Using SQL Server 2008, please.