Cutting The Line.
Three lessons from my post office.
1) Don't put a sign announcing changes at the entrance.
It seems logical to put it there, but people have been visiting the post office since they were young and they are not there to browse. They have a purpose and will purposefully walk straight past your sign because they know what they want to do and will not be seeking guidance. If you want to change their behaviour, a sign won't cut it. You have to understand their existing behaviour and adapt to it.
2) Don't dislocate human contact with your customer.
Yes, the queues/lines are a source of dissatisfaction. The solution is to reduce the waiting time not to displace it by getting people to sit down and wait for a number to be called. Moreover, a queue does give the customer some sense of connectedness with their goal where waiting for a number to be called with no indication of "time till service" is alienating.
(Side note - if you do have an indicator of "time till service", make sure it's accurate and make sure it counts down - unlike that in my revamped council office that identified me as the only person waiting for attention from a particular department and proclaimed my waiting time as 5 minutes, then 10 minutes, then 15 as the attention failed to materialise. Seeing your waiting time reduce is encouraging, seeing your waiting time being relayed to you is beyond annoying).
3) Don't berate customers for not understanding your new system.
Were a customer to stride past your sign on the way to buying some stamps and then stop bemusedly when assailed with an interior that looked like a doctor's waiting room with faux-leather banquettes scattered around, don't ask him if he's got a ticket. Ask him how you can help. And if he were hypothetically to ask what sort of stupid system this was, don't answer one in which queue jumpers are frowned upon. He might take offence at the implication, he might never darken your doors again and he might write to the chief executive about it. Hypothetically, of course.
The queue is never the real problem, the wait is.