One of the most common mistakes I see when out in the field with VMWare, is incorrectly configured VMWare ESX servers. It is very common to see people configure virtual machines as if they are physical machines. If you are going to implement VMWare in your environment I highly suggest going through the VCP training. The knowledge a VMWare instructor can share with you during your course can be invaluable. They can tell you about all those little things you just can’t find in a book.
In the mean time you can follow this simple guide from VMWare on performance tuning. It is quite helpful in guiding you along the path on how to get the most out of it. Check out this link:http://www.vmware.com/pdf/vi_performance_tuning.pdf