This is written for VM Workstation, but a lot of the ideas will work on ESX.
Virtual Strategy Magazine published a tutorial on how create and customize an optimized master virtual machine for creating several clones for work:
Many people have been asking for some tips on how to build a VMware Workstation virtual machine. The following guide has been written with the assumption that this will be the first virtual machine you will be configuring, and that you will be creating this as a template virtual machine.
A template virtual machine, will be a virtual machine, configured by you, which will be prepared for rapid deployment of additional virtual machines. Rapid Deployment means being able to deploy a new pre-configured virtual machine in 10 to 15 minutes.
An assumption will be made that you have the current version of VMware Workstation (as of this writing, version 5.5.1 build 19175) installed on your workstation. It will also be assumed that you understand how to install and configure Windows XP Professional, as this will be the operating system configured as the template.