Cardigan and Hand Bag

It's progessing,... The cardigan I mean. I'm almost at the arm pits now, and I have to take off of the needle to see whether it will fit or not. Meanwhile (because I couldn't push my self to take the cardigan off the needles) I knit a hand bag on my new 7mm needles. I used off white yarn (same as the beret. I got 4 pairs of bag handles a while ago, but only 2 are delivered yet. Hope the other 2 pair will arrive soon, since I bought new yarn today already. I'm knitting a hand bag. Yes again. The one I just made, I made a promiss that I would try to sell that one, and I will. But I saw this really nice colored yarn (it's coral color I think), and I just couldn't resist. I'm going to make a slightly bigger bag that the one I made already. Besides, the first one isn't finished either. I still have to sew the lining for that, but thats not that much work I think. And for my new bag, I got some hook to make a detachable strap for it. I'm just wondering, does anyone have an idea how to keep bag straps for stretching after a while? I was thinking of knitting some plastic string into it, but I'm not sure...