This is a bit dated (circa 2003?) but here is a tutorial we generated back at the University of Rochester for using a head-mounted and HMD-mounted ASL eye tracker, gives some good tips for those systems & basics on eye tracking in general
Bonus: Here is another on the Phantom haptic feedback system (i.e. robot arms that give force feedback so it feels like you are touching virtual objects).
Both of these pages are from the Hayhoe & Ballard VR Lab at UT. Unfortunately, the site is out of date, they actually have a totally new lab space with a more modern setup, but this covers much of the equipment I worked with between 2001-2012.