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1. Learn to predict. If you can't do this, you can't play competitive.
2. Learn to build teams with good synergy and coherence. Understand what mons work well together and what your team's general strategy is.
3. Understand that some mons can't cut it in the tier you choose to play. I love Malamar, but no set will stand up in OU when there's blatantly better options that don't require nearly as much set up to work.
Having said that, you can still do neat tricks and memes with decent to good results. I recently got to 1700+ in OU with a Kings Rock Cloyster of all things.
It's all about understanding what you and your opponent want to do, can do, and actually do.