Well, as they say, you're never too old to learn. If you were here last week you might have seen me being all angsty about gathering. I got some good suggestions and I was excited about the dental floss idea but for some reason I decided to try Marcy's suggestion first: always pull the bobbin thread. In 40 years of sewing nobody ever told me that. I don't think my mother knew that either because she always gathered by hand. Anyway, it worked. I gathered the skirt on the little floral print dress and the thread didn't break! Thanks, Marcy.
I'm the kind of person who is very embarrassed by and reluctant to admit to my mistakes and ignorance but if this can help someone else avoid 40 years of not knowing how to gather I can bear the shame.