Just figured out after 8 months that I was shuffling my deck wrong the entire time
A guy at our Tuesday night meetup in Fresno watched me mash shuffle my Commander deck for about ten seconds and said "you know you're just bending the same five cards over and over, right?" and I looked down and he was completely correct.
I used to think mashing was just mashing and it didn't really matter how you did it, but then a guy at my LGS showed me the same thing with my deck. He fanned the cards out and I could literally see the same chunk of lands and spells staying clumped together no matter how many times I did it. Now I split the deck into six or seven piles first and then mash those together, and my games have been way less flooded or screwed.