1. I took mine the summer right before senior year. Take it sooner if you think you may struggle, and will need to take it a next time (you'll know based on the reviewing you do). I'd recommend mid-late june, so that you have enough time to study well after your last finals. Earlier if you can fit it in, but make sure not to rush studying. Proper prep for the first may save you taking it a second time.
2. Start studying as soon as you get a chance (I started the week after my last final of that spring). I just find it easier to study for one thing at a time, so that you aren't crossing neurons accidentally (and end up thinking of leg muscles when you're trying to do algebra)
3.Definitely just once. Its like the SAT. Stressful, 3 or 4 hours long, just not something i'd recommend taking twice. Just go into it preping as if you only get one shot (but don't stress, cause you can always re take it).
4. I studied on my own. I'm ok in groups, but I find that i'm usually at some different level than the others. I'm not even saying I'm ahead, as I usually benefit at some point from someone else explaining things. I'm just better when I can go at my own pace, and not have to interrupt my thought processes or mod how fast I'm going.
5. I bought a 30 dollar or so Kaplan book. It had sections on writing, math, reading comp, the whole 9 yards, it even had a few 'top words' and tips on analogies, and a CD with really helpful practice tests (it actually predicted my scores very accurately. I also bought a Barron book with the top 800 words, and spent a whole month memorizing words I will never again use (or even recall, at this point). It was a complete waste! I saw maybe 4 words out of all 800, and I even used 6 in my essay to console myself for wasting time. If you're gonna try and study words, do word roots, not words themselves.
But keep in mind that your GRE will be different. I believe they are taking out analogies, and allowing calculator use. I can't say whether this will make it easier or harder. What I will say is DON'T take it now, as you might end up having to retake it (when I entered undergrad, they had just added the new SAT, so they were still accepting old versions. If they put in the new GRE now, 3/4 years from now, they probably won't be taking old versions, as everyone will have had ample time to take the new one.). Hope this helps, and good luck with your journey- it definitely pays to prepare now.