Step #1: place a physical barrier on your machine bed to guide your seam allowance. I like to use this product:
http://www.amazon.com/Dritz-Quilting...ive+seam+guide
To place it, I use my favorite ruler that has fine lines. I place it under the presser foot and lower the needle so the needle tip is just to the right of the 1/4" line (helps create a "scant" 1/4" seam allowance). Then I lower the presser foot to hold the ruler in place. Next step is to check that the ruler is running exactly front-to-back. This is pretty easy to tell by comparing the ruler lines to the throat plate edge. If necessary, I adjust the ruler so it is running exactly front-to-back. I remove the plastic from the underside of a Dritz adhesive guide and place it so it butts up against my ruler edge. (Lift and move if you don't get it right the first time.)
Step #2 is to use a fine thread. My preference is Aurifil 50wt 2-ply, but polyester 60wt thread will also work. Using a fine thread like this helps to give you that "scant" seam allowance.
Make a test strip using 3 pieces of 2.5" fabric. After you seam them together and press, they should measure exactly 6.5" wide. If they are less, chances are high that you are not pressing your seams correctly. This brings us to step #3. Make sure that you are not pressing so that there is a little extra fabric in the seam. This extra "turn-of-the-cloth" will add too much to the seam allowance so that your previous two "scant" seam allowance allowances (needle to right of 1/4" line on ruler, and fine thread) will not be enough.
If you adjust the above 3 factors until you get it right, and if you are careful when sewing to butt up your fabric edges to the physical barrier, you will not only get accurate seam allowances -- you will also be able to sew fast.
Trust me on this.
Incidentally, in order to do this on my machine and still have fabric connecting with the full width of the feed dogs, I have to remember to move my needle one position to the right every time I turn on the machine. To remind myself, I often place a piece of painter's tape over my sewing machine dial. The last quilt I did I sewed merrily without doing this the first night, and have been paying the price of the constant adjustments you describe ever since because my seam allowance was too wide!
Edit: Instead of a ruler, in the first step you can use 4-squares-to-the-inch graph paper. Place the needle just to the right of one block's lefthand line.