Every time i go mid or top (2v1)
When u get advantage, kill all creeps with last mana and go buy first item:
When u got money foranddont wait and go buy (if possible and no one pushing). Remember, u have advantage with exp gain!

When i get banshee i go gank/help others. Thats it :)
This guide for support Zilean (not killer), good cooldown helps a lot :)
GL & HF

(My eng is bad, so its very small guide)