Here is a link to the lightning safety guidelines published by NOLS:
NOLS Backcountry Lightning Safety Guidelines
If you don't want to read the whole document (which is an excellent resource), here are the Spark notes (no pun intended) from my own NOLS course. I believe the rule was that if the difference between the lightning flash and the thunder clap is less than one second, you get into "lightning position". Lightning position was basically squatting down as low as possible, backpack off, but with only your two feet touching the ground (don't let your butt touch the ground). The reason has something to do with the way the current would enter/leave your body in the event that you are hit. If you're with someone else or in a group, you should spread at least 50 feet apart (as it says in this link).
As described in this document, it's not a good idea to lie flat on the ground because it increases the chance of the current traveling through the ground and getting you that way.
Anyways, don't take my word for it, just read the info in the link. For what it's worth, we did have some serious close encounters with lightning and had to do the whole "lightning position" thing several times. I remember not being nearly as scared as I should have been (I think I was happy to have that 60+ lb pack off of my back for a few minutes, even if it meant potential death by electrocution--seriously).
Edit: Additional info about lightning position: your feet should be TOGETHER. This reduces the chance of the current entering one foot and exiting the other. Also, it says if you have a foam pad or soft clothes available, to "sit" on that. I think that means assuming the lightning position with the pad under your feet, but I'm not sure. It might also help to pray to Zeus.