Park in such a way as not to interfere with anyone, do not block traffic. This will not only show respect for others, but also reduce the likelihood that someone accidentally touches your bike, drops it or scratches it while trying to drive around.

Be sure to check that you have not left the key in the ignition. Also, don't put the key in the front "pockets" of the bike.

The next very important point - do not keep documents for the bike in the trunk! In the сase of theft, it will be impossible to prove your innocence. And also in the case of free parking, it is recommended not to leave anything of value under the seat (money, phone, documents, etc.) at all, since sometimes the bike can be opened. On many scooters, it is not difficult to bend the seat and climb into the trunk with your hand without breaking the lock.

It's not recommended to leave the helmet just hanging on the handlebars. In a paid parking lot, it will most likely be dropped if the parking guy moves your bike. Free parking is easy to get stolen if someone else likes your helmet. The best recommendation against helmet theft is to take it with you wherever you go (in a cafe, shop, etc.).
If you have nowhere to put your helmet or you are leaving for a short time, you should use a small cable with a lock for fullfaces; open helmets are best tucked under the seat or at least hide the strap under the seat so that you can't just pick up your helmet.

And one more recommendation for free parking: lock the steering wheel to make it more difficult to roll your bike (for the purpose of digging into the trunk, for example). Especially it's necessary in empty locations, and if you are potentially not blocking the exit, etc. And near the establishments where the parking guy works, on the contrary, he himself will ask you not to block the steering wheel, so that he can easily move the bikes to open the exit for other visitors.