What ever you would feel comfortable going to work or doing the shopping in will suffice. They are after people who will listen and make a judgement on the facts given in court, not someone who wants to enter a fashion contest. I have done two lots of jury service and both times just dressed in my normal clothes so i would feel comfortable if stuck there for ages. As Daffy says, the last thing they want is juror's fidgeting around because of the way they are dressed. It can be bad enough keeping your concentration at times, without any distractions, if you get a bad case/prosecutor/defendant!!