If you're giving your stitches a little tug after each one, don't. It's a common mistake that people make when they start out. Let the needles and yarn determine your tension--don't pull at the stitch once it's gone from one needle to the other.
Another possibility is that you're accidentally twisting your stitches. Make sure you go through the part of the loop that's on the front of the needle, closest to you. For knit go from left to right throught the front loop and for purl, from left to right.
Have you checked out the videos?