Place your heavy, and canned goods before your fragile, and crushable goods on the belt.
If you don't want the stuff you're buying destroyed by the time you get home, make it easier on the person bagging your groceries (usually the cashier) by placing all heavy, and canned goods on the belt first.
There's some freedom when it comes to alot of stores, because they have people who bag the items for you. But at many stores, it's the cashier's job to bag items for you. So place them on the belt in the order in which you want them bagged.
We often times have no choice but to just toss items in a bag regardless of the order in which we scan them. Only because we have to get you in and out as quickly as possible.
If you make it easy for us, we'll make it easier, and less stressful for you.