I have never made anything that could become an heirloom. (Possible exception - maybe my wedding shawl?) Generally, I make things so that I can use them in my everyday life. I'm not always super practical about it (see last post about superwash yarns...), but neither do I make tons of special occasion things that get used once and then put away. That's what I've always assumed one did with heirlooms, because how else would they last for generations? As it turns out, heirlooms don't have to be delicate fancy creations.
When my son was born I received some things that have turned out to be heirlooms, even if they were never intended as such.
Here's the Bean in a sweater my grandmother knitted for me when I was a baby, on a quilt that my other grandmother made for me. I wore the sweater (easy care acrylic, definitely washable) as a wee thing and I used the quilt on my bed throughout my childhood. He'll grow out of the sweater too quickly, I'm sure, but the quilt should last a while yet. Who knows if these things will pass down to another generation, but for now I'm glad to give them to my baby from his great-grandmothers.