I've been selected three times in different parts of the country. I suppose it's less than one in ten years. As it's random some people might never get called, and others called lots of times. If you've been on a very long case once you can explain and ask to be excused further duty.
It's just part of life's rich tapestry. As a citizen, it is your duty and does no harm to see how the "justice" system works. If you try to avoid it or talk your way out of it once, they will take care to catch you another time. If you tell the clerk you are self-employed and losing money, they will usually try to assign you to a short case. If you tell them you don't mind being assigned to a long case, you may or may not get one. there is a lot of sitting around so if you are in business on your own you can take in a box of your paperwork to do in the canteen. But if you are absent or don't keep alert and pay attention during your trials you can be very heavily fined or even locked up.
You get selected just by being on the electoral roll and catching the eye of the clerk in the court.
They might pick every 5th person on each 9th page, or any other "randomised" method.
I knew someone who once had this job and amused herself one Friday afternoon by picking out people with funny names.
Her game became obvious when they were read out for selection.
