I had this problem when starting out, and it is quite frustrating!
After reading and learning from ppl here, I sew a seam, and measure, sew a seam, and measure.
I know it sounds tedious, but until I become adept at being able to just look at something and 'wing' it...(I'm not holding my breath on that one, lol!)...this is my method for accuracy.
Let us know, what works. Accuracy is quite an issue in my cave :D