In the physical world, performance optimization is typically a factor of throwing more powerful hardware at the problem. Faster CPUs and more RAM usually lead to better performance. It's not so simple in the virtual world. The greatest strength of virtualization (shared resources) can also be its greatest weakness. The current generation of hypervisors do wonders to manage memory commitment and CPU scheduling, but they lack much on the storage management side. Disk contention due to lack of spindles on your storage array can cause issues that are not easily seen at the virtual server level. In order to optimize the performance of your virtual infrastructure, you need to peer deeply into the environment and identify the problem areas.
There are several vendors such as VKernel and Hyper9 that offer products designed to do deep analysis of a virtual environment and identify areas where you can tweak the environment to perform better. Even VMware's own CapacityIQ is progressing on that front. In order to take full advantage of your virtual infrastructure, you need to eliminate bottle necks and identify future pain points. Planning for growth will also help to keep your environment optimized to perform at its best.
One of the easiest and most highly recommend first steps in optimizing the performance of your environment is to ensure that the environment complies with VMware's recommended best practices. An in-depth analysis and Health Check can provide insight into the configuration of your environment with respect to VMware's published best practices. The VMware Guy routinely visits client sites to perform VMware Health Checks and overall Environment Analysis. Afterward, a detailed report which outlines deviations from recommended best practices is given to each customer. This report will enable you to remediate problem areas and tune your environment to perform at its best.