By now I'm quite efficient, haha.

Heavy stuff in the beginning, fragile stuff at the back, from large to small (so that I bag the heavy and large stuff in the bottom and then fragile and small I squeeze in carefully).

I recently heard of a tip from people who become anxious at the cashiers: Fruits and veggies at the end, because often they need to weigh it and sometime also input numbers manually, this gives those extra few seconds at the end.