acknowledging customers, even if you can't serve them immediately, is the best
way. Just a "I'll be with you in a minute" as you walk by to the till or to
grab a bottle works just fine - it lets the customer know that you realize
they're there, and you're just trying to fill the current order before you get
to them.
Also, if you have customers who are hesitating after you try and take their
order, come back to them - don't let them make you wait. Just a simple "I'll
be back in a minute when you have your order ready" usually works. Either
they'll fire it off right there, or you can serve someone else in the meantime.