The MAC of the virtual network card will not change between boots, so if your DHCP setup allows you to assign fixed addresses to interfaces this way that is the easy route. If you are using a cheap router's DHCP feature you might not have this option, but good routers will and if the DHCP server is a Linux or a Windows box then you definitely can. Will the most common dhcp server found in Linux distributions something like this in the config file will do the trick:
host somevirtualmachine {
hardware ethernet 00:0C:29:36:BC:91;
fixed-address 192.168.39.240;
}
To find the MAC of the network interface in a Linux, run ifconfig and look for the HWaddr for the interface. VMWare may also present the information on its management screens.
If you can not set a fixed address on the DHCP server then you could set a fixed address in the VM if there is part of your range that the DHCP server doesn't hand addresses out of (it is common to let DHCP set addresses for part of the range such as 192.168.X.2-128 and reserve the rest for devices that need fixed addresses). How to do that depends on your OS (editing /etc/network/interfaces on Debian style Linux setups).
You could also look into dynamic DNS, though if you are not setup to be able to give fixed addresses via DHCP then you probably aren't set to configure dynamic DNS locally either.
NOTE: You should give more detail in your question about your setup, particularly what OS is running in the VMs and the device responsible for handing out addresses via DHCP. I've assumed Linux in the examples above but it'll all work with Windows too (you'll just need to ask a Windows admin for details).
