Intresting article about how big ERP rollouts can go horribley wrong ,the information is a bit dated but the princilples are the same
Take-Home Lessons from Nestlé's ERP
1. Don't start a project with a deadline in mind. Figure out the project requirements, then determine how long it will take you to accomplish them.
2. Update your budget projection at regular intervals. So many things happen during a long project that you will be lucky to stay on target during a particular year, let alone the life of the project. Frequently revisiting your numbers will help minimize troublesome surprises.
3. ERP isn't about the software. It's easy to put a new system in place. The hard part is changing the business processes of the people who will use the system.
4. Nobody likes process change, particularly when they don't know it's coming. Include in the planning the people whose processes you are changing. Keep the communication lines open while the project is in the works, and measure the level of acceptance before, during and after the rollout.
5. Remember the integration points. It isn't enough to simply install new systems; you need to make sure that they can talk to each other.
info from CIO.com