Love all the comments - some more things to tack onto this conversation from a 2 page note from my local quilt club Amador Valley Quilters on "How to attain Accuracy" for our block of the month. Author Jan Steinhoff.
Touching on the things not mentioned in the prior post comments:
Fabric should be smooth, no wrinkles
Be consistent on the measuring (do you cut to the left or the right of the ruler marking every time??)
Don't speed when you sew. Don't stitch over pins - it can cause inaccuracy by slightly delaying the stitch, thus shortening it.
Do not use large needles and extra thick thread - the finer the better/more accurate.
Align fabric edges properly - if you are even a thread off, the seam will not measure accurately.
Press seams flat - don't iron (drag iron over fabric). Lift and put iron down flat. Set stitches by pressing seam flat first. Then roll it over so you press it open, Press down, lift up - don't grind iron down the seam.
Fun fact - in a 12" block with inch long components - if you are off by 1/16 of an inch in each component, you will be off by 3/4 of an inch at the end.