I've made a few of these. I agree that if you make a mistake and miss it for a while, it is hard to correct. One thing I made a point to look for as I did each row is to make sure the knit side had a single stitch at each end and groups of two all the way across. The purl side has groups of two all the way across.
When you do the knit after the slipped stitch on the front, make sure you're knitting into the first of the group of two. When you work on the back, make sure that your needle is going between the two grouped stitches for the first part--the p2tog.
This helped me catch any mistakes right away. Hope it helps.