A lot of it starts with the fabric prep. Cutting from fresh new smooth fabric seems like a no brainer. But new fresh fabric may both shrink and grow when you get around to ironing your blocks.
The other is knowing the perfect spot for the 1/4" seam allowance on the machine you are using. I probably have at least 50 9 patch blocks in the scrap bin, from working out that seam over the years, not much scrap value, but experience value, priceless.