The more and "better" you play, the system will think you dont belong where you are, and will grant you more LP.
When i started this season, i bombed my placements (just a shit fest with AFKs), i ended up Silver 3, i was gold 2 last season, made it to plat 5 in the preseason (QQ no rewards), i was able to skip Silver 2, and end up in silver 1, went knee deep in the mud and got to gold 5 in like 30 games, then after that, 150 games later, still gold 4-2 area.
The more you win, and the more often you win, while not losing, the more lp you will get.
Its a good rule to just only plan on playing 3 games, if you lose 2, dont play your third, take a couple hour break. If you lose, win, lose, try the next game, if its another lose, dont play, just take a break.
You also need to learn when you need to dodge, its better to lose 3 lp and not take any MMR loss from dodging a bad game, vs losing 20 lp and mmr.
A good example of a bad game is when the prelobby is already toxic, all premades, or if your team, vs their team looks like this.
(first is your team, second is theirs).
Mid:
vs
Top:
vs
Bot:
vs
Jung:
vs 
Just unbalanced teams, vs stacked balanced teams who have the damage, tank, and cc to outplay you in every department.