Cutting: Are you cutting accurately? Do you use the same brand of rulers for your project. Sometimes lining up your fabric on one side or the other of the mark on the ruler can make a difference. Don't cut too many layers at once.
Sewing: that pesky 1/4" is a misnomer. You don't measure the seam, you measure the resultant patch! Here's a good way to check:
http://www.quiltingboard.com/tutoria...ce-t89997.html
Also, your seam allowance is a moving target. If you change your thread weight or fabric type, you should recheck your seam and adjust if needed.
Make sure that you maintain your seam allowance at the start and end of each patch. It's easy to get off track at these 2 places.
Pressing: Make sure that you are not pressing a tiny crease in the seam, or stretching/distorting the block.
Square up your units as you make them, then square your blocks.
I'm sure others will have more suggestions.