Hi, Mummy: I can't recall where you live, but here I go out in the woods and get moss! It grows on old stumps and on the base of trees. If it has small ferns or other vegetation growing in it, all the better.
After lining the wire hanging basket with moss (moss side out), I begin putting the soil in a bit at a time, so that I can plug through the wire to have violas and lobelia growing through the sides of the basket.
Sometimes I weave small dead branches around the wire, or add sea shells, etc. It's wonderful fun, and much lovelier than the liners they offer for big dollars.
I used to have a friend in England who owned a nursery and she would do hundreds of baskets for the town displays. She always lined hers with moss first and then black plastic and as Baa commented, punched holes in the plastic. Then she would place a round of old carpet at the base. This still enabled her to poke the plants through the wire from the outside in. Her displays were wonderful and lasted the whole season even suffering neglect because they were in public places hung from lamp posts and such.