List of things good players often don't do because they are lazy.
Part A. Your Team:
A1. Team Composition
-Mage (AoE Nuker)
Jungler in team:
-Make sure there's a good allied champion for a 2v1 lane.
-Make sure the good ally TAKES the 2v1 lane.
-If you have a Blitzcrank, Alistar, Rammus, or Poppy on your lane, there are advantages to letting other team push early. Try to farm using last hits only.
A3. Summoner Spells
-Make sure someone has Rally
-Make sure someone has Clairvoyance (submitted by Fbsunny.)
A4. Aura Items
-Make sure someone builds Aegis. Make sure there's no multiples of this item.
-If your team has many physical DPSers (3 or more) then make sure someone builds a Stark's Fervor. Also make sure there aren't multiples of this item.
-If your team has many magic DPSers then make sure someone builds a Soul Shroud. Make sure there are no multiples of this item.
-If your team has many magic Dpsers then make sure someone builds Abyssal Scepter. Make sure there are no multiples of this item. (submitted by Fbsunny.)
A5. Other Items
-Make sure somebody buys wards. Vision wards if there is a Twitch or Evelynn on the other team, Sight wards otherwise.
-Make sure somebody wards the dragon and Baron Nashor when the time is right. Lvl 5+ for dragon and 10+ for Baron Nashor.
-Oracle potions on player with high survivability if there is a Twitch or Evelynn on other team. Possibly for Shaco as well.
A6. Reminder to team
-Ping before gank.
-If sufficient notice is given before a gank, let your lane be pushed toward your tower. This way you have more room to chase.
Part B. The Enemy Team:
B1. Team Composition
-Check Ratio of Magic to Physical Damage. Declare to your team the preference for armor and magic resist if the ratio is lopsided.
-Check for Area of Effect damage and crowd control spells. This will determine your formation. If there is a lot of area of effect spells, then choose a loose formation.
-Check for healers. If there are 2 or more healers, make sure somebody builds Executioner's Calling later on.
-If the enemy has a jungler then be aggressive on the 2v1 lane, the usual plan is to use brushes to harass hard. Try to let enemy team push in beginning and keep attacks on minions to last hits only. Even if you have AoE spells for farming minions such as Sivir's Boomerang, Corki's whatever, don't use it until your opponent dies.
-If the enemy has a Shaco who jungles. Harass at their lizard at start.
-If the enemy has a Fiddlesticks who jungles. Harass at their golem at start.
B3. Summoner Spells
Check for the following:
Keep track of who has the following:
-Smite - Very likely to be a jungler.
-Flash - Harder to gank, wear down before kill. Target those without Flash for early ganks.
-Ghost - Harder to chase, wear down before kill.
-Exhaust - Extra dangerous opponent.
-Executioner's Calling - Hard on healers.
-Check for low defense and health. Rank the targets.
-Check for high damage. Rank the targets.
-Check for Oracle Potions and Wards.
If you would like to add anything, please post and I'll give you credit.