If you've had a bad day, leave it at home and be polite while on shift, even to the meanest customers. You don't need to be a pushover, but you're more likely to leave your customer happy and satisfied if you're slower but in a great mood than if you're the fastest cashier ever but snappy and rude. If you can't sincerely be cheerful, at least try to fake it. Actually listen to their response and acknowledge their response. When they are walking out of the store, they will feel properly acknowledged and will likely come back to your store. If your store has a protocol to call for backup when the line gets too long, call for backup and try not to rush through the entire line yourself. Customers feel uncomfortable at best and disrespected if cashiers keep chatting around or over them while being served. This is your job, and just as other people don't chat away at work constantly, neither should you. Learn to drop and pick up conversations when it is appropriate.
ابتسم وكن لطيفًا. اسأل الزبون "كيف الحال؟" وتمنَّى له يومًا سعيدًا. اطلب الدعم. توقّف عن الحديث مع زملائك بالعمل عندما يحين وقت الخدمة.