Repeat the order in your head a few times. Also while making one order, take
the next order, then give the "knowing look" to the third customer waiting.
Simply make eye contact and give a brief nod- most people get the message,
this means "you're up next, get your order details ready". Also, if people are
taking to long to pay or scrounging around for coins go onto the next person
and keep serving while keeping one eye on the people who are yet to pay.
Serving people in the order they come to the bar is not always easiest,
sometimes it's best to just serve from left to right. If you do it right then
no-one has to wait long anyway. And while taking an order always be doing
something with your hands such as wiping the bar top or clearing empty
glasses. The key is efficiency, efficiency, efficiency! Good luck!