Good bag packing is a two way thing. I detest customers who just throw everything on the conveyer and then complain about all of the frozen things being kept together. I always try and keep like-things aside so they can be packed together but it becomes impossible if everything is spread out. Also, if you don't want certain things with other things (that most people don't mind having together), let the check-out person know - we're not mind readers.
I always try and fill the bags up, though I try to make them not too heavy - especially for pensioners. Try and keep delicate things like bread and eggs in the same bag - it'll save you a lot of complaints. Learn as many of the fruit and vegetable codes as you can, it's much quicker than looking them up. Try and pack things vertically, I find you can fit more in. Playing tetris also helps with neat packing.