The Shared Services and Outsourcing Network has an interesting article on the top 10 mistakes made when implementing a shared service center. They are:

Not measuring costs or service levels before a move to shared services
Not documenting processes and work streams pre-implementation
Not appointing a full-time head honcho early in the process
Not focusing sufficiently on the transition period
Not having a robust project plan clarifying employee resources
Fighting the battles of yesterday, not those of tomorrow
Becoming bogged down standardizing technology and processes pre-implementation
Believing that “it’s already a centralized process: there’s nothing we should do”
Having no, or inadequate, risk management or monitoring
Omitting the "make versus buy" equation

I'll comment on various aspects of this article in future posts, but for now I'd like to discuss the first point - not measuring costs and service levels prior to the move to shared services. This is a huge mistake because every business case I've ever seen for Shared Services talks about how much cost savings there will be and how much better the service delivery will be. Unfortunately, if you don't have baseline performance metrics for the effectiveness and efficiency of your delivery, there's no way anyone will be able to substantiate how much cost was reduced or how much service delivery has improved.

I'm a strong believer in regular baselining of costs and comparing that performance to external data as a benchmarking exercise. This should be a regular part of your operations, but it should certainly be done in advance of a move to shared services.

The article is long and I've only posted a summary here. Here's the link for the full article:

Top Ten Mistakes When Implementing Shared Services