Here are a few of my favorite tips/tricks for seam allowance.
I like the idea of testing your seam allowance by stitching three strips together, as Scissor Queen suggested. I use 3-1/2" x 1-1/2" strips. The center strip should then be exactly 1". If it's too big, your seam allowance is too small. If it's too small, your seam allowance is too large. Sometimes, it's only off by a thread's width!
If you are measuring your seam allowance to see if it's 1/4", be sure to count the stitches as part of the 1/4".
PRESSING can also make a difference!! If you are pressing from the wrong side of the fabric instead of the right side, a little fold forms at the seam allowance on the right side, and you'll lose real estate. Press the seam flat to sink the stitches, then press on the right side, getting the fabric as close to the stitches as possible to flatten the seam, without popping the stitches.
Water/Too much steam can also make a difference! Remember, it's cotton, and even if you've pre-shrunk your fabrics, if you get it too wet or use too much steam when pressing, it's still going to shrink. Press fabrics prior to cutting, then use a dry iron and a light mist if necessary when piecing.
Cutting can also make a difference! Cut using the marks on the ruler if possible, and try to use the very same ruler to cut out all your pieces. Make sure the line on the ruler sits on top of the edge of your fabric when cutting strips.
Here are more precision piecing pointers:
http://www.rileyblakedesigns.com/cut...-tricks-tools/