Division is by far the easiest way.

The method I use on my plant nursery is to cut back the parent plant hard (ie remove all or almost all of the old the foliage) after flowering. Wait a few days until small new foliage appears and then divide this up using a spade or knife.

If you don't cut back hard first you'll usually find there's too much leaf on the divisions for the divided plants to cope with and they'll wilt badly.

You don't have to wait until after flowering but it always seems a shame to cut off flowers.

Early September is a good time to do this as the plants will develop new foliage before winter and have the winter to re - establish themselves and will grow away strongly in spring