In the Readers Digest Complete Guide to Sewing in the chapter about hand stitches they describe an "Arrowhead tack".
I find that I use it all the time. Most of my skirts have inverted pleats at the back and in order to prevent the top of the pleat from coming undone I sew the arrowhead tack at the top of the pleat. This used to be done many years ago and somehow is not fashionable any more.
There is always a fair amount of strain at this point and the tack certinly does help. I use embroidery cotton.