1. Never rage/get angry or respond to ragers. If you succumb to your emotions your decision making will definitely take a hit.
2. MAP AWARENESS. This not only helps you, but your teammates. You can warn your teammate from the imminent gank(to you), instead of saying "wow, that was so obvious why did u stay n00b?!"
3. Mechanics. Every action you take must be the most efficient one. harassing between cs, not missing cs, harassing when the opponent will inevitably go for cs, etc. a lot of this is laning prowess
4. Leadership (or attempt to) Instead of just thinking to yourself, "wow, this is probably gonna end badly.." man up and spam back pings, command in chat, lead by example.
5. Wide knowledge of champions (both knowing how to play and how to play against) this one just comes with experience and practice.