Sourcingmag.com has an interesting case study from Les Mara, the head of HP BPO in EMEA.  In the article he discusses HP's journey in Shared Services, the benefits and the lessons learned.  Below is an excerpt from the article showing the "lessons learned"  that I thought was particularly relevant: 

1. Understand why you are pursuing a shared services/outsourcing model -- It's important to develop a compelling business case for moving to a shared services/outsourcing model. Whether it's to reduce finance transaction processing costs or to increase operational efficiency, a company needs to have a clear idea of what a shared services and/or outsourcing operation will do for them.
2. Secure buy-off from stakeholders -- The shared services/outsourcing concept cannot be imposed upon the business units. Leadership within the company must obtain buy-off from the individual units. It takes an investment in governance and stakeholder sponsorship in order for shared services or outsourcing to succeed.
3. Anticipate change -- This also brings about an organizational change, therefore leaders need to develop a clear communication plan targeted at the various stakeholder groups. This allows the company to anticipate and manage risks proactively.
4. Invest in training and recruitment -- Roles and responsibilities change in this new model. Companies need to invest in training and recruitment of team members on the internal resources for their new shared services or outsourcing responsibilities.
5. It's a continuous journey -- Shared services/outsourcing is not a journey with a beginning and an end. This is a continual process and HP is still on this journey and reaping the benefits along the way.

Note how the five points focus strongly on the human element.  While there are a number of factors that go into creating a successful Shared Service Organization, little of it will matter if change management is overlooked.  The extent to which key stakeholders and the organization's employees embrace the move to Shared Services will largely determine how quickly and successfully the business case for Shared Services is achieved.

Here's the full article:  HP's Journey to Shared Services Finance & Accounting Success