Okay, this mainly uses common sense but I've never seen this posted and I think most guilds don't use this but it can really help with DPS if you're going for hard-mode achievements. The way the beam works for those who are not familiar is: In phase3 of the Yogg'Saron encounter, a purple beam comes out from the boss which reduces your sanity by 4% per second while you are facing Yogg'Saron. Normally ranged DPS just turns around effectively stopping damage for ~5 second at a time...however there is a way around that.
Assuming the raid positions itself behind Yogg'Saron (everyone will be in the northern quadrant). Now keep in mind with normal game mechanics, you don't have to be 'facing' your target completely in order to cast/attack them. So position yourself around the north-eastern side (north-western works also) and face your character so your back is to Yogg'Saron. Have your tanks tank the guardians a little farther away from Yogg'Saron and you're all set...whenever he uses the purple beam you shouldn't be hit even once and you won't have to stop DPS on the guardians. Along with not stopping DPS, this will also completely negate the need to go to the sanity wells at all in P3 (-Freya anyone)?
Note: Keep in mind that if you have a spell that turns your character to face the mob you're attacking, you can hold down your right mouse button facing away from Yogg'Saron and it won't turn you.
Here is a picture showing where I stood when we did 1 light in the darkness.
Enjoy, if used correctly - this will almost certainly help your guild with Yogg'Saron and it will never be fixed since it's 'working as intended'.