you need to play chess every day or you get rusty and blunder a lot. playing blitz seems to make your OTB game worse. but this might not apply to everyone. solving puzzles help a lot. chess is really hard game, even for the very best players, and even the best chess engines sometimes can't solve a simple mate in 2. so take on the challenge!
the two quickest and most effective ways to improve in chess is to study endgames and study the chess of grandmasters. i prefer bishops over knight in most cases. a pair of bishops is as strong as a pair of rooks, and stronger than bishop+knight, which is stronger than 2 knights. in correspondence chess games, i try to make the best move, but in OTB games, I just try to get the opponent to make mistakes. most players are very strong in tactics, but have never study the endgame, so i try to simplify to the endgame early to one that is in my favor. Rook and pawn endgames are especially important. Endgame study is really tough but the most rewarding as it can get you the most wins.
Studying the Opening beyond the first 5 moves is overrated. In correspondence games, you can think up of your own opening moves as you go along, because you can analyze a lot of variations. In some games where you are allowed to use the help of a computer engine, you can definately make figure out good moves that may be superior to just looking up a database. I think only at the highest levels of OTB play is Opening theory memorization important, so just know the general principles and save your time.