Runes:
Marks - never change
Seals - depending on your opponent you can go with flat armor seals for physical damage champions or switch in more health per level if you feel mana isn't a problem
Glyphs - personal preference for scaling ability power because I play very aggressive post lvl 6, scaling magic resist is another viable option if you're worried about mr
Quints - Movement speed is another option if you feel you need it for dodging skill shots however in most team fights your ult provides more than enough mobility and the flat ability power gives you a punch

Items:
- Abyssal Scepter should can be picked up against heavy magic damage champions such as LeBlanc.
- Rush Zhonya's after your dorans if you're again a physical damage assassin such as Zed.
- Athenes or Morellonomicon can be picked up if you really like to spam out spells
- Sorcerer's Shoes can be purchased early for better mobility but its really up to you when you feel comfortable getting it