Matthew McGiffen shares one technique to generate reference numbers using a sequence and the
One thing to note is that, while the sequence will generally produce unique number, it is still worth enforcing that in your table definition with a unique constraint i.e.
ALTER TABLE dbo.Orders ADD CONSTRAINT UQ_Orders_OrderReference UNIQUE(OrderReference);
This prevents someone from issuing an UPDATE command that might create a duplicate reference.
As long as you can live with the occasional gap in your reference number, sequences are a good solution to the problem.