Don't play when you are tired or distracted.
Keep your focus on the game.
While playing especially do not think about how much rating you'll gain or lose with the current game, just think about your moves.
Don't play when you don't really feel like it, for the only reason to (re)gain more rating points. Only play when you're in the right mood for chess.
Don't underestimate lower rated players. Always give it your best.
Don't try to win super-fast or in fancy style. Don't play for the gallery. Play solid and normal moves, even if it takes a long endgame to bring in the harvest.
Patience!
The most important thing is to try to avoid mistakes as much as possible and not to repeat them! If you lost, do take the time to review the game and not only look at what the engine suggests as best move, but really try to understand the position. What's going on on the board, what where you thinking and why did you make that mistake? What was the problem with your move, how did your opponent refute it? Were there any warning signs in the position that you should have payed attention to?
Don't just think of your own moves, threats and ideas but also of what your opponent can do. Try to figure out why he played the moves he played. What are his intentions?
Strive for activity and try to gain/keep the initiative as much as possible.
In worse positions do not ever lose hope or mentally resign too early. Keep on fighting. Not just playing for time, but actually looking for a swindle. Maybe a stalemate. Or that famous draw with lone king against king, rook pawn and wrong-colored bishop. Be creative. Once the position is very bad you haven't got anything to lose anymore so you can try anything. Do you know how hard it is to win with K+Q against K+R? Even the checkmate with K+B+N vs K isn't trivial. Then women's champion Ushenina couldn't do it against Olga Girya some years ago; look it up on YouTube:
www.youtube.com/watch?v=YFF5ibgB6eASo try to escape into such endgames, hoping that your opponent doesn't know how to win them.
Whatever happens on the board, try to keep your calm. Your opponent played a surprising move and now it looks like you're lost? Take your time, look into the position. Maybe there is a refutation, a zwischenzug, anything you can use to save the position.
These are just some generally recommendations that come to my mind quickly. For anything more specific you will need to study your games, especially you losses in depth.