Do not install 1.5.x.x or any lower opencart version. Use 2.3.0.2 or the latest stable version
Do not install vqmod
Do not install too many SEO extensions. Always use SEO extension from one particular developer
Avoid using Nitro Cache if your website is frequently updated with products or price or have plans to install many other extensions. Nitro cache module has been a nightmare for many opencart users
Never edit or make any changes to core files or template files. Always use OCMOD modification for any changes.
Take frequent backup of database and files and save it locally and also on server out of the public_html folder