Masteries, Runes and Items
If you are queuing up for blind pick as a carry type champion (damage dealer/non-tank), you would probably do best to spend 21 points in the offense tree , focusing on the primary damage type your champion deals, while supplementing it with 9 points in defense, grabbing the extra magic resistance since you will often be facing a champion that deals magic damage. However, if you choose a safe champion that can farm from range and avoid most damage altogether, it is often a better idea to spend your final 9 points in the utility tree in order to gain more mana regeneration and increased buff duration.
As for draft pick mode, you will usually be able to guess which champion you are playing against based on what champions have been picked by the other team. In this case you should always use masteries that supplement your damage output with defense attributes that combat your lane opponents primary damage type. Again, if you are playing a safe, distance farmer like Karthus or Orianna, you could forego the 9 points in defense for more sustain from the utility tree.
Finally, if you are playing a tanky type of mid laner (Cho’Gath, Galio, Maokai, etc.), you should probably spend 21 points in the defense tree and then do your best to decide if you will need more sustain from utility, or damage from the offense tree for your final 9 points.
Runes for the mid lane follow almost the exact same pattern as masteries. You should balance between runes that amplify your own damage, while countering your opponents primary damage type with armor or magic resist. There are a lot of possibilities here, so we suggest you take a look at the guides for specific champions and specific matchups to determine the best option in any given situation.