A little tip: 5 AM is the best time. I got in right away. Of course, by then, you might see that what you want is full, but the same results would have occurred if you just sat there and clicked again and again.
If the class is full, and you really would like to take it, I suggest emailing whoever it may concern, and asking them to let you in. I fought to get into World Religons (in a polite manner, though), and it got to the point where the lady actually emailed me back asking which tutorial I would like. she asked for my student number and opened up a spot for me.