I've seen some people explain this on YouTube, and I have to agree with these statements.
"Don't play a game to win; play a game to improve." This is important so even if you do lose the game, make sure you take something out of it to better yourself and make you win more games in the future.
"Never give up." This follows closely with the next statement,
"Be positive!" No this doesn't mean your K/D (also as a pet peeve I have, I hate people comparing your skill to your KD. You could just be having a bad game or the other person happens to out-skill you. That doesn't necessarily show your skill or experience with the game/champion.) Be a positive player and keep your teams hopes up. If you are the person that's saying "GG Team too heavy" or anything related to that, you are lowering your team's morale and making that game feel utterly hopeless. What you should do is keep your team's spirits high by complementing people whenever you make a good play (even if its a small thing such as warding, and warding especially in lower elo/rank) and also make sure to reassure your team after something bad happens. Something as simple as "It's ok guys, we can still do this." can help make you and your team more energized and positive, leading to a possible comeback.
As an extra thing to remember, no one is perfect, not even the pro teams. There is ALWAYS something you could have done better. If you come across a bad play that you and your team made, don't yell and flame at them. Instead you could offer a suggestion in a POLITE way such as "Doing x wasn't a bad idea, but maybe next time you could try y and see if it works better. Just offering a suggestion." However, an even better thing to do is to just worry about improving yourself (still doesn't mean putting down your team in the process, but focus on improving yourself more than your allies) since after that game, more than likely you won't play with them again.
If there's anything to take away from this ramble (aka here comes the TL;DR)
Be Positive
Don't give up
Always strive to improve (on yourself mostly)