For me the most important thing was to have fun.
Having fun had the biggest impact on me. If i had no fun while playing, I would do poorly and also... what's the point?
Other than that - I suppose playing while hungry, sleepy or in a bad mood isn't a good idea.
Game wise, for me, it was important to know my deck and possible combos and pay close attention to the map, as there is and never was fog of war - a huge strategic advantage for both players. Don't let the enemy overcome you with an unseen well in the corner of a map, as noticing it early may give you an offensive advantage quickly.
My favourite strategy was to push hard on 2 bases at the same time. This way, it is possible to switch focus and make an enemy waste Power on spells and defensive units.
Base switch early on was a very difficult thing for me to handle, as I was more of a T2 - T3 kind of a guy, T1 was my early side.
And, one last thing. When in my lower ranks, I built a up-to-T4 deck. Don't. Do. It! For your own sake
Good luck