A couple of bits of advice to add to Merrick's brilliant posting, which I agree with wholeheartedly.
Re: the issue of 'bad trips'...
Like Merrick I'm a bit of a fan of the 'uncontrollable' aspects of LSD and the freefall weirdness of it all. Like I said earlier, I've never had anything I would class as a bad trip. However, for those who are worried about this there are two things you should remember if it starts to happen. Firstly, remind yourself forcefully that you have taken a drug. This is not the permanent state of your mind, and it will subside. This is sometimes enough to calm you down and prevent you from going with the freak-out feeling. Secondly, if you feel unable to change the situation you are in yourself in the way Merrick suggests (lighting, music etc), agree with the friends you are tripping with to use the word 'Freak!' in an emergency. (Do this before dropping the tab, and make sure EVERYONE knows this.) What you can then do if you feel things are getting too much is to say the word 'Freak' to one of your friends, and they will leap into action. Using this word avoids stringing a sentence together and trying to express in words how you feel.
This code word has been an unwritten agreement me and my friends have had for years. It's only needed to be used twice. The one time I was involved one of my friends came up to me in the club we were in, said the word and I leapt into action. I whisked him out of the club, far away from the music and madness and mayhem and he calmed him very quickly. He later said he felt that having this word was akin to a mountain-climber having a safety rope.