Preparing for a virtualization deployment at the physical layer seems easy, but many times strategies quickly falter. Many organizations buy hardware and expect it to handle any workload. For example, sizing a server with a SATA drive array or a low-end SAN for multiple virtual machines (VMs) with moderate to high disk I/O requirements will result in poor disk performance for all VMs on that host. Examine your VM workloads first and architect a host server to meet those demands and to allow for the natural growth of resources.
Virtualization also introduces new processes for installation, management and disaster recovery (DR). The ability to deploy VMs in your new virtualized infrastructure will not suffice. Inevitably, problems will arise that necessitate recovery steps. Virtualization enables easier DR, but only if you are familiar with the products and trained on the available tool sets, which differ significantly from those for physical environments.
Bottom line: Know your workloads and study new tool sets before starting a virtualization deployment.