Stop giving it away
I'm going to make a gut check here. If you're losing, it's because you are giving the game away, so stop it. Don't make it easy for the enemy team to extend its lead.
Right at the start, if you're losing your lane in the laning phase, don't engage. If you are down even one kill or more than 30 last-hits, it's time to start thinking of an alternative battle plan. Don't engage unless a roaming ally is in the wings. Think for a minute on the last time you engaged. You got wrecked, and now your opponent has experience and items that you don't. Will you really be able to win a fight at this deficit?
A long time ago (back in Season 2), I was playing Vayne with a Leona support. I got one early kill, based, and bought a Wriggles' or something. When I came back into lane, the enemy support tried to get aggressive on me. I retaliated with a double kill and continued to lane. My enemies respawned and again, trying to make something happen. Leona caught the enemy carry, and another pair of kills were fed to my lane. I pretty much won the game singlehandedly after this.
I am not hyping my ability to play ADC here; I'm not a very good Vayne player. However, it is completely ridiculous to believe you will fare better against a Vayne with 300 more gold and a level on you. Even if my opponents aren't playing as well as I am, I treat them as if they are dangerous. I'm not going to lane against Lux and pretend she's not going to properly combo, even if she hasn't landed a combo on me all game. I'm going to wait for her to hand the lane to me before I take it from her. I still count her cooldowns; I still look to juke her snare.
In the midgame, if you are losing teamfights, the answer is probably to avoid them. There are three possibilities: Either the enemy got ahead in kills during the laning phase, the enemy's team composition is better at midgame teamfights, or the players on your team suck at teamfighting.
Don't try to start a fight if the enemy has an advantage; this is asking to lose. Start fights in advantage situations instead. If they're pushing towers, fight near your towers. If they're going for the dragon, attack while the dragon's DPS is helping you. If they're warding or clearing buff camps or pushing, group up and pick them off. Don't fight fair because if you're behind, a fair fight ensures that your team will lose.
If your team as a whole hunkers down and stops throwing the match away, you will eventually catch up. Kills on higher-level characters give more experience, and breaking killstreaks gives more gold than individual kills on your team. Be patient, clear jungle creeps, push back waves, and get in kills when you can. Don't try to bring the fight to the enemy until you've got a lead.
Wards win games
I am not experienced enough in Summoner's Rift to write a good guide on warding, but I can say with the utmost certainty that wards win games. Spotting an enemy player roaming gives you a fair amount of perfect information and a ton of imperfect information. It tells you where at least one player is, and it tells you where his friends might be. If you're sly, you might be able to position your team to cut his friends off or isolate him and kill him before they arrive.
At the intermediate Elo rankings where I tend to reside, people use wards, and this means that you will have to do something about that. Send your support, top, or jungler to clear wards with an Oracle and put more up on your side. Because you're behind, you may want to have your entire team covering your ward clearer; you can stage ambushes because your opponent knows you have a support running around clearing them. If you know that they know, you also know that they want to kill your Oracle. If the enemy doesn't respond, you clear their wards for free, and if they do respond, you have the element of surprise.
Once the enemy wards are down, ward all the main jungle exits and entrances. Warding is not just the support's job, even if Sightstone will probably be in her inventory. Pick up some wards, a Wriggles', or a Sightstone, and back her up. If you can keep tabs on where enemies are likely to invade your jungle (and they will), you have the chance of cutting them off. Additionally, they'll need to pick up an Oracle to clear your team's wards, and you can potentially kill it and waste the gold they spent on it.
Although it's a bit underrated, Clairvoyance helps a lot in lower-level games, especially if your map sense is good. Pros can use very little information to come to strong conclusions, but we plebians need to have map information spelled out for us. Using CV to identify whether the enemy is waiting to ambush you can make a really big difference. I would run CV more in my games, but I usually ADC or jungle, neither of which can really fit it in.
Start working together
Let's face it: If your team members were actually helping each other, you probably wouldn't be losing. It's possible that your team is a cohesive unit and the enemy team is playing better despite that, but chances are if you are reading this guide to learn from me, the enemy team is a bunch of random people. They're probably not working together like clockwork. Even at the super-top levels of ranked play, there are a lot of games with minimal teamwork.
This is something that is hard to get people to do, unfortunately. If you ask your team to actually go in when you initiate, it sounds like orders. A lot of people don't respond well. Be positive and supportive, but also try to direct your team with a good battle plan. Be very careful when you communicate and try never to be negative. If you have 1% chance of winning, a negative attitude from one player on your team can bring that to 0% very quickly.
Ultimately, you're not going to win many of those lost games. I advise surrendering as a rule because it's better to replay the parts where you lost (usually the laning phase) until you get it right. Still, sometimes you get games where you are down and you come out ahead. Just remember that your opponents are human too, and they have every chance of throwing away the game as you do. Good luck and have fun!
We understand what it's like to climb the skill ladder in League of Legends. The Summoner's Guidebook teaches you the tools you need to get a competitive edge. Whether you're climbing the ranked ladder, playing Draft Dominion, or getting crushed by intermediate bots, every enemy has a weakness. And every Thursday, Patrick Mackey shows how you can improve improve on yours.