Whew...okay, after 7 of the most painful hours of my life, I finally got the damn Venus Sigil and I hereby resolve to NEVER go back to the Thunder Plains ever again. But anyways... for the purposes of this thread, I figured I'd throw in my two cents about how it's done. For me, I stayed in one spot at the mouth of the path leading away from the Rin's Travel Agency in the Northern half of the area. There, the lightning hits about every 3-5 seconds and there aren't any lightning rods nearby to get in the way. I turned off the FFX sound (as it really does become a distraction) and focused my vision on the body of water you see in the upper-right corner of the screen. This way, there was nothing in my line of sight to interrupt my concentration as I waited for the light flash. Using my peripheral vision to detect the lightning, I went about the task, and almost a third of a day later, I emerged with Venus Sigil in hand. I'm not sure how successful you'd consider this methodology to be, seeing as it took me the better of the day to finish this incredibly stupid side quest, but I thought I'd propose another plan of attack to those readers out there still suffering through this ordeal.