We've just been and couldn't buy tickets - only advice I can give is to get there early, probably at least half an hour before, and once the queue starts moving it does go down at a pretty decent rate so don't be afraid if it looks a little long
Try to look on the website a day or two before the day you're looking to get tickets for. I was trying to book the 22/3, and it was already full by January. I checked on the 20/3 and it showed availability, so I used the computer at city hall and booked & printed tickets whilst there.