How to eat an elephant
In my experience there are a number of things that can at least improve the chances of an enterprise-wide BI initiative succeeding. It sounds like motherhood and apple pie, but the project needs business sponsorship if it is to succeed; IT is rarely in a position to drive such a project. However that statement on its own is of limited help. How can you find this elusive sponsorship?
The place to start is to find the people that actually have the problem, which is usually either the CFO or the head of marketing. The CFO has the job of answering the questions of the executive team about how the company is performing, so knows what a pain it is to get reliable numbers out of all of those bickering departments and subsidiaries. The head of marketing is the one who most needs data to drive his or her business, usually involving looking at trends over time and involving data from outside the corporate systems, and this is usually poorly provided for by internal systems. The CEO might be a sponsor, but often the CFO will be carefully feeding impressive looking charts to the CEO to give the impression that finance is in control of things, so the CEO might not be aware of how difficult data is to get hold of. The head of operations or manufacturing is another candidate, though this person may be too bogged down in operational problems to give you much time. If there is someone responsible for logistics and supply chain then this is often a fruitful area. Sales people usually hate numbers unless it is connected with their commissions (where they demonstrate previously unsuspected numerical ability), and HR usually doesn't have any money or political clout, so marketing and finance are probably your best bet.
So, you have a sponsor. The next step is to begin to sort out the cross-enterprise data that actually causes all the problems in taking a holistic view, which is these days being termed master data. If you have multiple charts of accounts, inconsistent cost allocation rules, multiple sources of product definition or customer segmentation (and almost all companies do) then it this is a barrier in the way of your BI initiative succeeding. There is no quick fix here, but get backing to set up a master data management improvement project, driven by someone keen on the business side. Justifying this is easier than you may think.
In parallel with this you will want a corporate-wide data warehouse. Of course you may already have one, but it is almost certainly filled with out of data data of variable quality, and may be groaning under a backlog of change requests. If it is not, then it is probably not being used much and may be ripe for replacement. To find out, do a health check. There is a bit of a renaissance in data warehouses these days, and these days you can buy a solution rather than having to build everything from scratch.
In truth your company probably has numerous warehouses already, perhaps on a departmental or country basis, so it is probably a matter of linking these up properly rather than having to do everything from the beginning. This will enable you to take an iterative approach, picking off areas that have high business value and fixing these up first. Once you can demonstrate some early success then you will find it much easier to continue getting sponsorship.
In one of the early Shell data warehouse projects I was involved with we had a very successful initial project in one business line and subsidiary, and this success led to a broader roll-out in other countries, and then finally other business lines came willingly into the project because they could see the earlier successes. This may seem like a longer route to take, but as noted by Robert Farris, this is a journey not a project, and if you start with something vast in scope it will most likely sink under its own weight. Much better to have a series of achievable goals, picking off one business area at a time, or one country at a time, delivering incrementally better solutions and so building credibility with the people that count: the business users.
Elephants need to be eaten in small chunks.