Think about security when positioning the till. Theory has it that the best location is just inside the entrance way to the left, so that customers must walk the whole way round the shop before making their purchase. In reality you will need to site the counter so that you can stand and view as much of the shop as possible. Having it right next to the door may also help to deter shoplifters but I like to have it a little further back into the shop.This is so that genuine customers do not feel intimidated when entering the shop by having to make instant contact with the shop staff.
Ideally you should be able to see every nook and cranny but in reality you cannot see through customers, so decide which areas are the priority