Try to use the same ruler, or at least the same brand ruler for the whole project.
Here's a good way to check your seam as you sew, this will also work with the ruler you selected, and check your cutting:
http://www.quiltingboard.com/tutoria...ce-t89997.html
Make sure that you are also pressing your seams correctly. Sew, Press (up and down) on the seam you've just sewn, then press (up and down) the seam to the side. Make sure you don't have a tiny tuck at the seam.
Measure each block as you go in case you are drifting off. Till you get the hang of it, don't wait till you've done 5 blocks to get a measurement.
And finally, don't be too hard on yourself. It's great to shoot for perfection, but the quilt will be lovely even with some slip ups.