How do people in cafés carry coffees without spilling them?

I recently started work in a shop that sells coffees. I’m pretty fine with making them, but have difficulty carrying them out to the tables. We have to fill them up until they are peeping just over the rim of the cup or mug, and then get them to the customers as fast as possible.

I’ve tried one hand, two hands, and on a tray. On a tray has worked best for me, but it’s a little bit inconvenient to have to use a tray to carry only one coffee. Does anyone have any tips about how to hold the coffee, how to walk, etc?