Disclaimer: everything I know about accurate piecing I learned from Bonnie Hunter.
1. Place your ruler so that the measure line is ON the fabric (fabric edge showing past the ruler's line) not NEXT to the fabric. If you are confused by this let me know I will post a picture. This extra couple of threads will allow for the fold when you press the seam. I think you are already doing this when you say you overcut.
2. Make a test seam, then measure the size of the resulting unit. For example when you sew 2 strips that are 2 1/2" wide together the finished strip should measure 4 1/2". If it is smaller, adjust to make your seam narrower. If larger, make a slightly wider seam allowance. Recheck until you find the sweet spot. Remember - it's not important to have a perfect 1/4" seam, it's a perfect unit size that you want!
3. Before sewing the units together, make sure they are the size they need to be. The more pieces in your block, the more likely your block will come out the wrong size (usually too small).
All the other tips given are good too, but getting those pieces to be the right size before you assemble the block is the key. Good luck with your quilt!