I have a trick I use to make sure that my stranded knitting doesn’t tighten up too much.
First, of course, I try my best to knit a little more loosely than normal on every stitch.
Second, whenever I change from one color to another, I put the needle in the next loop, and then make a SPECIAL effort to stretch out the stitches on the RIGHT needle. Here are two pictures:
I actually stretch it a little more than this, but this is what I can manage with only one hand.