1: Accept that you're going to lose games. It happens to everyone, and in a 5 person team game there's going to be the ones that just get away from you.
2: Have a rain maker or three, and know their in's and out's **** well. Since her remake I've almost never lost with Evelynn, when I'm not forced into mid by process of elimination and can actually pick Ahri as a counter instead of picking blind I almost always win with her, and otherwise I can just do work with Soraka.
3: Don't have a lane or role you just can't do. Alternatively don't be adverse to queue dodging when you're stuck with one you can't do. You shouldn't be playing ranked if there's a job you flat out can't do, and you shouldn't take blind pick seriously to begin with. It just comes down to luck most of the time.
4: Communicate. Let folks know what's going on. Don't be the guy who's the last pick who waits till the last second to say he can't do job X. Give your jungle, or have an idea as jungle of which lanes are going to be your good and bad lanes.
5: Have ~6300 IP on the fly. No, seriously. It's not at all unusual for Riot to leave characters over powered for months on end- Riot almost completely ignores unpopular champions like Rumble even though he has a statistically high win rate. Evelynn was ignored until she became popular. Malphite wasn't getting touched till he started showing up every game. Right around when Vlad, Sona and Miss Fortune were all new they were all "ban me or see me picked by the other team" tier characters, and they were broken as hell.
6: Make sure your picks make sense. Akali is a good character for disrupting combo-driven characters. Kassadin is a strong anti-caster, but only against high volume casters like Cassiopeia. Don't pick Soraka to play an extremely aggressive bottom lane. Don't pick Leona to play passively. Even at low ELO's counter picking counts for something. It's complicated but you shouldn't be the guy thinking Singed is a great counter pick for Nasus as long as Nasus keeps chasing him.
7: Arguing helps nothing. You can turn around some stupid kill deficits if you get your act together. You can't argue your way out of what already happened. You can do something about future situations.
8: If it's not working, stop doing it. If you're blind pushing your lane for the fourth time, and getting killed for it the fourth time, it's time to back up and think.
9: Wards are helpful, but actually being arsed to counter ward wins games. More junglers get shut down because their team never wards, or counter wards, and the other team is always on top of it. Yes, even if you're hugging your tower 24/7. The advanced warning on what the enemy jungle is doing- especially if they intend to dive- is incredibly important to winning disputed lanes.
10: You do not need to ask permission to buy wards. Even AD carries will find themselves with ~75 gold to spend and not needing it for, say, health potions. Supports have enough on their plates and you just look like an ******* for expecting them to bankrupt themselves on wards. If you're *****ing about ward placement, especially if the enemy team has a character your team has been ignoring that has Oracles, or just being active counter warders, its your own **** fault. A lot of games get decided right around the half hour mark because one team's support suddenly becomes ineffectual because they only have a healthy line of GP5 items.
11: On that note, plans are useless, but planning is indispensable. In one sense, some supports are naturally useful with a minimal amount of gear. Some supports are also called Karma. Some teams are designed to win early, some wont bloom till past the 30 minute mark. Don't be the team surrendering at 20 minutes because you have half the kills a team with three soaks, a tank and a carry have. Expect it, and adapt.
12: The best character is whomever you can play well. I've carried games with AP mid Soraka. I've seen Ezraels jump 5 people at Baron and net a pentakill. I've seen characters with almost no damage potential win the game for no reason other than that the enemy team wouldn't stop focusing them.
13: Nut up or shut up. If someone is annoying you, ask them in as polite a manner as you can muster to shut up.
If they don't, you have the ignore function. If its really that bad, your team wont feel any compulsion to report you so much as captain assface. Don't be the guy who AFK's in the fountain because someone's a belligerent *******. It's not fair, but you have to be the bigger man (or woman) over it. Reacting to it only makes you the bigger *******.