Use an identify value in the database, and a computed column to generate the whole item.
Assuming you have an Identity field called Ident, a fixed text field call Prefix, and a DateTime field called InsertDate:
[ID] AS (((((([Prefix]+'/')+CONVERT([varchar],datepart(year,[InsertDate]),0))+'/')+CONVERT([varchar],datepart(month,[InsertDate]),0))+'/')+CONVERT([varchar],[Ident],0))