For me, it starts with making sure my cuts are all the same. Being off 1/16" or 1/8" can really add up. As important for me is making sure I have a precise 1/4" seam. Then I make sure to nest the seams that come together, and I match up each row/intersection to make sure they are even, then I pin, pin, pin. As I sew I then check and double check and usually have fairly good luck. I also use the seam guide on my machine, and extend it out with tape to keep everything straight.