I was reading Steve Maine's blog entry on object-oriented business layers and it hit me that I've never seen a decent article on the use of the Flyweight pattern on the business layer. I think this especially applies in large relational based implementations and I've also found the visitor pattern useful in some situations where the OO-Relational mapping becomes hairy or the task complexity is high.
One complaint about OO models is that they solve the “constant-data, varying behavior” but fall down somewhat on “constant behavior, varying data”. But in a relational transaction-based system the needed OO model is utilizing a small number of classes/instances to act upon large numbers of data entities and flyweights handle this very nicely. Google searches turn up nothing on this approach, I'll have to keep looking...