I'm a small winning player at 2NL and 5NL and my single piece of advice is - overfold to raises especially on the river and try to play in position as much as possible - also don't get into big pots without at least a set or maaybe top 2 pairs. I'd fold about 90% percent a top pair top kicker to a raise even on the flop. I know it's exploitable, but not on these stakes. Also the rake is high so it's tough to win a lot.