Coming from a slightly different perspective, but I regularly Clerk School Appeal Panels and am also responsible for training Panel Members.
One of the first posters stated that he / she had never been called up by the LA, despite being on the list and having been trained. It is a fairly issue and it usually happens because the LA already has a large number of volunteers and these long-serving individuals tend to get priority. I don't particularly agree with this as it is always nice to get a fresh perspective from someone that hasn't "been in the system" too long.
Once you are trained, don't be afraid to contact the administrator who has responsibility for putting together panels and let them know that you would like to sit. Don't assume you will automatically be called up.
As I said, I'm coming from a slightly different perspective, but most Panel Members seem to enjoy the work, despite it being unpaid. You should at least be given refreshments / food, particularly if the appeals go on for an entire day.
Whilst most authorities won't pay expenses, make sure you don't have to pay for parking. We had a long-serving Panel Member who had, without us knowing, been paying to park in the public car park every time they attended a hearing. We actually had free spaces reserved for Panel Members.