http://www.parktool.com
Park makes 2 different types of chainring bolt tools. 1 is angled while the other is basically just like a screwdriver. Both feature a little "tip" that extends into the chainring bolt. When you reinstall the chainring bolts, it helps to put a little grease on the threads. Keeps them from seizing up. Just get them as tight as you can by hand. I've never had one loosen up or back out on its own.