These instructions will show you how I made my platform, if you make an improvement please share it with the rest of the Reprap/Makerbot community so they can give it a try.
Note that there are actually two sets of instructions here. The first set requires you to have access to metalworking tools and some basic metalworking skills. If you have a friend who can help you with this (like maybe someone in Robotics Club or who is a welder), that's just as good. The second set gives suggestions on how you can work around not having fancy/expensive tools.
This project requires a number of tools that can seriously injure you or a bystander. Wear your safety glasses and be careful.
A heated build platform uses Nichrome wire and can easily reach 220F/105C without any change in physical appearance.
Never leave a Makerbot unattended while the build platform or extruder are at temperature.