As your lane conflict comes to an end (usually through the destruction of one side's tower), you will suddenly find that you have a lot of options as to how to proceed. Before you can make a good decision, you need to be aware of the difference between safe and unsafe places. Listen to your "map sense".
-Safe areas of the map are nearby friendly towers or champions, or are places that you can escape from easily.
-Unsafe territory is anywhere where there is or could be an enemy champion. If you can't see enemies on your minimap, you should always assume they're nearby!
At the start of the game, the map is cleanly divided into safe and unsafe territory: your side and theirs. As each side's towers fall during the course of the match, your side becomes less safe and theirs more so. The map becomes more and more uncertain.
I said it yesterday, but it's important enough to repeat: wards are NOT optional! When you place a ward, you are gaining certainty over an entire section of the map for a whole three minutes, as well as insight into the enemy's movements. If you see the other team walk past your ward into your jungle, you now know your jungle is not safe. Even if you don't see them, you have eliminated several possible enemy positions.
Better map vision means better plays!
-----Farm or Roam?-----
"Farming" means focusing all of your attention on killing minions and jungle creeps, instead of picking off enemies and starting teamfights. If you are a character who needs more items to be effective (i.e. a carry), focus on farming before anything else. If you had a hard time in lane and are behind on your build, farm twice as hard. Don't engage enemy champions, and be ready to book it if they come for you.
However, you must know when to stop farming. Be ready to help your team secure important fights and map objectives. Remember, you are much weaker when split up! It won't matter how long you farmed if you can't use it to help win fights.
The bottom half of the river is home to Dragon, the main early game map objective. Killing dragon gives your team 975 gold (190 each + 25 for the last-hitter) and gives you a big advantage over the enemy. A common beginner mistake is underestimating the importance of Dragon! You should have Dragon warded as often as possible throughout the game.
Beginning around level 7 or so, start looking for opportunities to claim Dragon. A good time would be after killing the enemy bottom lane or jungler. Get your teammates' attention, then have bot and/or mid lane push their waves to the enemy tower and come to Dragon. You'll generally need 2 or 3 people. Make sure the area is safe; watch your minimap for enemies wise to your plan. Ward the surrounding jungle paths and place a vision ward (if you have one) to kill any enemy wards. When you're ready, have your tankiest teammate hit it once, then walk back to leash it into the river (this is usually safer than fighting in Dragon's little cove). Unload everything you have on it - be quick about it!
Dragon respawns exactly six minutes after it's killed. You want to remember exactly when this happens so that you can retake it ASAP! So when you clear it, look at your game clock in the upper-right-hand corner of your screen, mentally add 6:00, and type it in your chat box (if we get it at 14:35, I'll write, "20:35 dragon"). If the other team got it or you're not sure exactly when it was cleared, give your best estimate ("~21:30 dragon"). If you keep time, you can better coordinate your team to stay on top of it.
If you see the other team starting Dragon and yours is in good position, you should contest it. Even if you can't kill their team, you might still be able to stop it! Oftentimes, you and the enemy will be stuck in a "steal the bacon" situation, where Dragon is unsafe for both teams, but neither can allow the other team to take it. There's a certain... pas de
There are two jungle monsters on each side of the map that carry the powerful "neutral buffs". Killing them grants you their buff, which lasts 2:30 (3:00 with the Runic Affinity mastery) or until you die. If an enemy kills you, they gain possession of it.
-Crest of the Ancient Golem ("blue") - This buff gives the holder a large chunk of Cooldown Reduction and regenerates their mana (or energy) at a high rate. There are few characters who don't like wearing it, but it's especially useful on caster-types who are more limited by their cooldowns and mana.
-Blessing of the Lizard Elder ("red") - The auto-attack of a character wearing this buff will slow the target and deal extra minor damage over time. It's most useful on auto-attacking carries and junglers when they're ready to gank.
There isn't too much to say here. The neutral buffs are pretty important, so protect yours to the best of your ability and try to steal the enemy's when you have the chance.
-----Baron Nashor is Your Bestest Friend-----
Baron Nashor is the "boss" of Summoner's Rift that spawns in the upper half of the river starting at 15:00. Killing him grants every member of your team 900 XP and 200 gold, plus 100 gold for the last-hitter. In addition, every living member of your team gains the Baron Buff:
-Exalted with Baron Nashor - a unit with this buff gains +40 Ability Power, +40 Attack Damage, and a large increase in HP and MP regeneration. Lasts 4:00 and is not transferred upon death.
A team wearing this buff will have a very large advantage over their opponent. If you see the other team wearing it, be prepared to defend your base!
The rules for taking Baron are largely the same as those for Dragon: keep it warded, pick a good opportunity, and do it quickly. However, since killing Baron takes much longer and leaves you more vulnerable, you need to be extra sure to keep the surrounding area warded. Failure to do so could allow the enemy to steal the last-hit or trap you in Baron's cove. As with Dragon, knowing the right time to engage and when to shift your attention is key. When it's killed, mark its respawn time at seven minutes after.
A well-coordinated team can move to claim Baron as early as 20:00. Don't forget to ward it.
Teamfights are the meat of this game. The success or failure of your team depends on many factors, but here are three main points:
-Know when to fight and when to run
If your team doesn't have a reasonable chance of winning teamfights, you simply shouldn't fight them. Defend your towers and farm until you can. Consider using a poke strategy, backdooring, or split-pushing the enemy towers, but don't instigate a fight. Stay safe and don't get caught. If your teammate gets caught, leave them behind unless you're absolutely sure you can help; otherwise, the enemy will just kill you both.
If your team is strong and ready to fight, group up and move together. Look for good opportunities to initiate fights or catch enemies when they're split up. Punish bad positioning. Force them to contest you at map objectives (Dragon, Baron, and their towers). Plan ahead of time which characters and skills you're going to initiate with.
-Have your kill priorities in order
You almost always want to focus their squishy carries first. Ignore their tanks as best you can. Protect your own squishies with your tanks. Pay attention to both your own positioning and that of your opponents.
-----Press Your Advantage!-----
If you're ahead and winning team fights, it's critical to capitalize on your success. If you don't, you're just allowing the enemy more time to catch up. Don't forget that the goal of the game is to kill towers and inhibitors, so shove them down whenever you get a good chance. Always put pressure on the enemy! A few strategies you might use are:
-Poking - Keep your whole team together and use ranged abilities without engaging a full-blown fight. An effective poke team will force the enemy back to base to heal without ever actually fighting.
-Split-pushing - Separate your team into two or more groups and push multiple lanes at the same time. The opposing team will have to either split up or choose a tower to lose.
-Backdooring - Have four team members defend towers while your fifth quickly pushes alone. This forces their team to respond to a single person, giving you an opening.
Keep in mind that knocking down enemy towers will allow your minions to get closer to their base, making it easier for them to farm. If you don't continue to pressure them, you're just making their comeback easier. Don't let that happen!
I think that's pretty much everything you need to know about mid- and late-game LoL. If anyone has something else to add, please post it in the comments! :)