The title says it all. It should be one of the first things you learn when you started. However, I have seen a lot of experienced techs who seldom (if ever) do it.

When connecting to a tank, purge your hose by cracking the center connection at the manifold for a second.

When connecting to a system, crack each connector for a second and then tighten. (Unless you are using ball valves or low loss of the same refrigerant type.)

Low loss brings up a different subject. If you are using them, you need to make sure not to switch between refrigerants without recovering all the refrigerant from your gauges.

Both air and incorrect refrigerant are not acceptable to add to the system, even in small quantities.

—Bryan