Just a word of warning, although you should digest all the above advice, DO NOT change your lifestyle DRASTICALLY before you turn up for your tests. I thought that because I was a bit of a boozer, I should give up completely a week before my very first aircrew medical. When the results came back, my liver had let out some chemicals that suggested I was a hardcore boozer and/or druggie!! Apparently something to do with your liver being used to it and then making some chemical to counter not having alcohol in the blood, haha. If you have a normal, healthyish lifestyle you really shouldn't worry about it. If you DO have something to hide, then I would stop whatever it is now, before it is too late!