Count your stitches.

And measure. I forgot to. I was in the rhythm of stockingette and didn't take time to count the stitches in my sock decrease...and when I did I had gone to far. Then I measured the inches under the arm of my tshirt, only to find out I'd knit 5 inches rather then the 2.5 before shaping. So rippit rippit rippit. A huge reminder to not get lulled into the rhythm of stockingette.