Taking root cuttings is an easy way to propagate herbaceous perennials – and now (late autumn or early winter) is the ideal time to do it as the plant is dormant. This method works well for plants that tend to produce suckers from their roots naturally, many of which have thick, fleshy roots. So, it is a good way of propagating Acanthus mollis, Phlox paniculata, Primula denticulata and oriental poppies for example. This is the method I use to prepare root cuttings from Acanthus mollis; it couldn't be simpler:

Lift a vigorous plant (I often try and take root cuttings from suitable plants when I buy them, i.e. before planting in the garden!). Clean the soil off the roots. Aim to remove no more than a third of a plant's roots and replant the parent plant as soon as possible.