The biggest mistake I see with people who are long time players of league is that they haven't adapted their play style since they started.
Unlock your camera.
Turn on Quick cast
Proceed to suck for the next 20 games while you get used to it.
If you like top lane, learn jungle.. a lot of the champions can easily translate into junglers and vise versa.
Start watching higher level players (LCS, twitch streams, youtube channels). You can learn a TON of tricks and tips.
Play consistently. If you spend a month off the game, you will loose some progress. Even if you just toss in an early morning aram once a week or something.... keep it going.
If you are doing all of these things, I don't know... I would have to see 4-5 games worth of gameplay to tell you what to watch for.