I use Sage software for my accounts, including customer and supplier invoices and payments. Each client has there own section in a folder and all their details are in there. Appointments are kept in my laptop.
www.manage-my-horse.com is designed for yard owners. It's not perfect at the moment but it's only a few months old and is the most sophisticated system I've seen that is designed specifically for yards. They update and make it better quite frequently so have a look.