I had this exact same problem the other day. I was sewing two lights with a dark inbetween. The lights were from the same fabric. 1 1/2" strips width of fabric. One of the lights was a half inch longer after it was sewed!
Saturday morning I was watching Everybody Sews with Sue Houseman (sp?). They were making a quil as you go in strips using batting strips. Sue said she ironed her fabric to the batting and it stayed in place while quilting.
I got to thinking that it might work with strips. And it did! I layed the strips together on the ironing board. Made sure everything lined up and then I pressed it with a hot iron. Don't move the iron back and forth, just pick ip up to move it and press it down again in another spot until the entire lenght is pressed. You don't want to stretch it.
I was surprised by how well it stayed in place and all the strips were then the same length!
Happy sewing!