1. The longer your team takes to set up, the worse it ultimately is. Baton passing works occasionally as a fun gimmick, but a smart player will just switch in their set up sweeper while you're baton passing and love you tenderly.
2. Defense always beats offense. This is something that isn't quite apparent at first and seemingly changes with the metagame, but the underlying principle is always the same: there are very few Pokemon who can straight bust through a wall unless they're mixed attackers or hit you SE (or you let them get off two Swords Dances in which case gg scrub). If they have a sweeper in and you have a good wall in as a counter, you have the advantage. Wail on their frail scrubmon.
3. Understand your battle plan before fighting. This should be obvious, but you shouldn't be going into battles saying "well we'll see what happens and I'll wing it." No. Bad. You will get rekt. Understand your team's roles and type coverage, because the better you understand how your team works at a fundamental level, the better you'll be able to use it. This comes with a second benefit of improving your mindgamez, because if you understand how your team works, you'll be better able to understand how opposing teams work and learn how to bait and such.
4. Experiment... but not too much. Instead of being a spaz and playing with fifty different teams, try setting up three or four to initially play with and tweak them slightly as you see fit. If you switch teams too often, you'll have a poor understanding of each individual team, relating back to point 3. If you start with a base and tweak it, you'll gain a better understanding of your team, and by comparing results you'll learn what effective team compositions are and aren't. I would say you shouldn't alter your team for at least five games after you do it once. Let things settle and don't let fluke matches throw your intuition.
I played a lot of DPP back in the day, so I can't really give you any meta-specific advice.
