This guide is applicable only for KVM based VPSes, for OpenVZ based VPSes please click here
To know the gateway and netmask of your IP address, please see this guide http://wiki.crowncloud.net/?viewing_network_gateway_netmask
Edit the interfaces file,
nano /etc/sysconfig/network-scripts/ifcfg-eth0:1
Add the following into it,
DEVICE=eth0:1
BOOTPROTO=static
IPADDR=IP_ADDRESS
GATEWAY=GATEWAY
NETMASK=NETMASK
ONBOOT=yes
Once the interfaces file is updated,
ifup eth0:1
Should activate up the interface with the new additional IP
To add more IPs,
in the files