Today, Getting On Board, have published a useful guide on How to become a Charity Trustee which I'd recommend for anyone who is considering a role as a Trustee for a charity. It demystifies a lot of the process and the legal duties involved.
I also note it is Trustee Week from 2 - 6 November, and they have a Festival of Trusteeship happening where you can join for free various session being run. It is worth checking out their agenda - even for existing Trustees.