Clematis does well with an annual pruning to encourage fresh growth. However, different cultivars require pruning at different times of year. It's important to know exactly when to prune your specific cultivar, since you can damage the plant if you prune it at the wrong time of year.
Plants that bloom on old wood, meaning the flowers appear on last year's stems, require no pruning except to cut back their size a bit and keep them contained. After they flower, prune back to a pair of healthy buds. (Apple Blossom is in this group.) Plants that bloom first on old wood and again on new wood, meaning the flowers appear on last year's stems and new spring stems, need to be pruned to remove weak growth. Prune them in early spring, before they bloom, to remove weaker stems, then again after they bloom to improve their shape. (Nelly Moser and Ernest Markham are in this group.) Plants that bloom on new wood, meaning the flowers appear only on new spring stems, should be cut back to 12 inches at the beginning of spring. (This includes Niobe, Princess Diana, Jackmanii, and Venosa Violacea.)