Update command that is selective based on passed parameters

I have a table "Tags" with the following columns
TagID int NOT NULL
BinSize int
OrderID int
AllocatedOrder int

I need to update these records, but I don't know in advance which columns to update.  Is there an elegant way to pass in parameters to a stored procedure and update only the columns associated with the parameters that are not null?  

That is, if I pass in@TagID =100,  @NewBinSize=null, @NewOrderID=25,@New AllocatedOrder=null it will update the OrderID of the record corresponding to TagID=100, but leave the AllocatedOrder and BundleSize columns unchanged?

Thanks.