Ignore those books that promise that some clematis don't need pruning at all. Pruning increases flowering, and not just at the ends of the vines (most of which eventually grow over your head, putting the flowers out of your sight); it also stimulates the vine to put out more shoots from the base, so the entire plant becomes fuller and more floriferous.
Start by taking out any dead, diseased, or damaged wood. Then follow the pruning directions below, depending on when your clematis blooms.
Pruning group A (or 1)
A is for After bloom
Includes: Species that bloom in early spring, such as C. montana, C. armandii, or C. macropetala
When and how to prune: Don't prune until after the flowers are finished. Flower buds were formed the previous year, so pruning before they flower means no flowers that year.
Pruning Group B (or 2)
B is for Before bloom
Includes: Species that bloom in late spring/early summer, including most large-flowered types
When and how to prune: In spring, cut back to a set of live buds, about a third down from the top. Hard-prune (to about 12 inches) for the first two years after planting to develop a strong root system.
Pruning Group C (or 3)
C is for Cut back hard
Includes: Species that bloom in summer/early fall: C. viticella, C. tangutica, C. virginiana, C. texensis, C. crispa
When and how to prune: In early spring, cut every stem to 12 to 18 inches or so.
Mail Order Sources for Clematis
Brushwood Nursery, Unionville, PA; 610-444-8083, gardenvines.com
Chalk Hill Clematis, Healdsburg, CA; 707-433-8416, chalkhillclematis.com
Completely Clematis, Ipswich, MA; 978-356-3197, www.clematisnursery.com
Garden Crossings, Hudsonville, MI; 616-875-6355, gardencrossings.com
Joy Creek Nursery, Scappoose, OR; 503-543-7474, joycreek.com