Okay - I found that both my Langmuir (Mountaincraft and Leadership) and my Mountaineers (Mountaineering: The Freedom of the Hills) have pretty good advice about lightning. I would advise anyone planning on heading out into the hills to read both of these excellent books - Langmuir is the book for British Mountain Leader Training, and the Mountaineers covers a lot extra information for activities over in North America.
I don't think being away from the treeline is particular problematic (unless you are the only thing on a very flat piece of ground).
As long as you are not on the top of a ridge/hill, then there is a safe "shadow zone" on the side of the hill - I think something of the order of 2/3 of the way between the hilltop and its base. The only thing is - you should sit on top of a plastic sheet or your backpack to prevent picking up stray eddy currents from the ground.
From Langmuir (Mountaincraft and Leadership, 3rd Edition):
You can get a shadow from a 7m or taller cliff where the safe zone is between 3m from the base out to the height of the cliff - this means standing 5m from a 7m cliff should reduce your chance of a lightning strike (note - reduce - there is still a chance you can get hit, there is no 0%). There is a diagram on page 227 of 3rd Edition - I can't find one to link to, unfortunately. He doesn't say how steep the cliff should be, though.
If you enter a cave for shelter (or dive under a boulder) - you must have at least 3m head room (so - 5m total) and 1m to either side.
From The Mountaineers (Mountaineering: The Freedom of the Hills, 6th Edition):
These guys are pretty clear:
If thunderstorms are forecast:
- Do not camp or climb in a narrow valley or gully
- Do not plan to climb or hike in high, exposed areas
- Watch small cumulus clouds for strong, upward growth
- Keep track of weather reports
If you spot a thunderstorm:
- Get away from water
- Seek low ground in an open valley or meadow
- Move immediately if your hair stands on end
- Do not stand under trees, especially in open areas
- Do not remain near metal or graphite equipment
- Insulate yourself from the ground with your pack or a foam mat
- Crouch to minimise your profile
- Do not lie down
- Gauge the movement of the thunderstorm