|I usually sew my buttons on with a zig-zag stitch and the feeddog down on my machine. Once in a while there seems to be a thread that, when you pull it, takes all of your stitches out.
On the last blouse I made I changed one thing. After sewing on the buttons, I now put a drop or two of fray block on the thread and - so far - this has worked. It seems like this will keep the threads from unravelling.