• Set your machine to 18-20 stitches per inch.  The smaller stitches will keep the seams from separating
• Don’t tug on your fabric as it is being sewn.  Letting the machine do the work of pulling it through will reduce or eliminate any possible stretching of the fabric
• Alternate sewing the strips from top to bottom. Sew the first pair from top to bottom, and then the next pair from bottom to top
• Try using a walking foot
• Sew shorter strips together – either cut the strip in half so you are sewing 20 inch strips instead of 40 inch strips; or begin with fat quarters (which are 20-22 inches wide).
• Sew the length of the fabric (parallel to the selvedge) instead of the width of the fabric (cross grain).  The cross grain tends to have more stretch than the lengthwise grain of the fabric.
• Start by sewing pairs of strips together. When you have all of the pairs sewn together, then sew pairs of pairs together (2 sets of pairs, resulting in one piece, 4 strips wide). Next sew pairs of the sets of 4 together. Continue in this fashion until you have as large a piece of "new" fabric as you want.
• try sewing without pinning the strips together

When I am sewing several strips together to create a "new" fabric, I sew two pieces of fabric together, and then sew the pairs together, continuing to sew the new units together until I get the piece of fabric the size I want.

I also alternate sewing from top to bottom and then from bottom to top. It may be a bit tricky if your strips are not all the same length. You can easily trim them to be the same length.

Once you have "new" fabric, you can cut it into strips and sew those strips together to create an even more unique look to your fabric.