I took a 2-day class with Harriet Hargrave a while ago, and one of the things I learned changed how I quilt.... the goal is NOT so much to get 1/4" seam allowance as it is to get the right sized block! No one will ever see your seam allowance after the quilt is made. If you have to make your seam allowance a bit more or less to get the correct size finished block, then do it. I taught a class yesterday and put these tips on the board for students to try when making their 3-strip test patch (sew 3 strips of 1 1/2" wide fabric together, press and measure to see if center strip is exactly 1" wide):
1. Cut accurately, use tape to mark the line on your ruler if you are not confident with rulers yet. Use the lines on the ruler, not the grid lines on your mat for better accuracy. Creative Grids makes great non-slip rulers.
2. use a 1/4" piecing foot, making the seam a scant 1/4" by not shoving the fabric too far to the right
3. move the needle one tick to the left or right to help achieve the desired seam
3. use a single-hole throat plate
4. use Bottom Line or So Fine bobbin thread TOP and bobbin (50 or 60 wt. thread creates less bulk in the seam and is just as strong as heavier wt. thread according to Bob Purcell, owner of Superior Threads)
5. use a smaller needle such as #70/11
6. use thread catchers and chain-piece your assembly
7. press (not iron) your blocks, setting the seam first, and be very sure NOT to have an tiny pleats of fabric in your seam. Use a light spray of starch or Best Press
8. square-up carefully
9. when doing cross-sewing on pieced blocks where you are going to be sewing over seams, use your walking foot instead of 1/4" foot
10. always nest and pin seam intersections
11. once your accuracy improves a bit, make your stitch length 1.9 or 2.0 instead of the default. This prevents the end of seams from opening up as you work with a block.
I hope these tips work for you. Keep trying! Let us know how you do.