How to increase ELO practically: Pick a champion and master it, you will gain elo eventually.
How to increase ELO theoretically: In order to gain elo, you must have higher than 50% win rate at the present. This means that if you win 100 games in a row and after that your win-loss rating goes to 50-50%, you cannot gain elo. This is also true in the other side of the spectrum; if you were 1800, lost 20 games, then go to win 50-50, you cannot get back to 1800.
Just to give my situation as an example:
173 wins - 137 losses - elo rating 1837.
Win rate: 56.2%