Depends what your personality is.
If you are a shy introvert, then you need public speaking practice such as standup comedy, or hosting singing jams, or doing improv, or open mics.
If you are an angry extrovert, then assertive comes easily, however, its a fine line towards aggression. Therefore you need to work on removing yourself from situations that prompt your anger such as removing yourself from egotistical, young and cocky people.
If you have positional power, there should be recognition by the subordinates to your position. It's up to you to convince them that you add value to the team through your boss-skills of communicating effectively, understanding the direction your business is going, your role in supporting that business, expert-level knowledge of the processes, people and things the natural work group uses everyday, and stuff like that.