One of the most important skills to learn in Yu-Gi-Oh
a Mulldrifter combined with a Baneslayer Angel, as Card Kingdoms own Michael Rapp put it
Opponent Tracking Keep a list of your opponents and view your match history and win rates against each one
Kotodama
Colorless Many players will find the card draw in their colors is more than enough to keep the cards flowing, but there are some reasonable colorless choices, particularly if your deck cares about artifacts
How often do you cast a turn 1 Serum Visions, look at the two cards from scry, and think to yourself, I have no idea if these cards are good or not? You can, and likely should, blame yourself for utilizing your spell poorly, but not using that mana has a cost too and with an instant-speed cantrip, you can make a more informed decision while not wasting that manathe best of both worlds