Clematis vines are classed in pruning groups. They convey the time and amount of pruning necessary to promote good growth and the most flowers. Group 1 is an early season bloomer, group 2 is midseason and group 3 is a late bloomer. Only group 3 grows flowers off of new wood and your other two groups grow from old wood. This means that timing is everything in regards to pruning. If you prune the first 2 groups before they bloom, you will not get blooms the next season. Group 1 is a spring bloomer; group 2 is a repeat bloomer and group 3 blooms in late summer or fall. This is important information to determine your clematis vine's group if you have lost the tag or cannot remember.
First year plants need special pruning to enhance the flush of growth and stems and to make a thicker, more compact plant. All varieties should be cut back in late winter to early spring to just 5 inches from the soil. Sadly, in groups 1 and 2 this means you just cut off the flowering wood but it will make a nicer, bushier plant in the future and make life much easier as you train the vine. Group 3 plants will not be affected by this harsh trim as it blooms off the new shoots produced that season and has until mid to late summer to grow these shoots.
Annual trimming of all three groups can be beneficial, but it is not as necessary in group 3 except for continued training. In the second year, trim groups 1 and 2 in late winter to early spring back to 3 inches from the ground. This remaining 3 inches is technically old wood so you will get some blooms but the stems won't get as tall. This is the step that gives you a nice thick bushy plant with prolific blooms for the rest of the vine's life.
Once these training cuts have been accomplished, yearly pruning is less extreme. Group 1 plants are healthy if you just remove old flowers and thin out wood that is not producing. Continue to train the plant onto or over whatever structure is supporting it and cut out errant stems. Group 2 vines can be left unpruned but they have a neater appearance if you remove the spent blooms. Cut them off to just before the next bloom node. These are the largest flowers in the 3 groups and the easiest to maintain. Group 3 plants benefit from thinning and deadheading but don't necessarily need deep pruning unless the vine has been neglected and needs rejuvenation. In all cases, rejuvenation will require you to prune back to 5 to 6 inches from the ground and then start training all over again.
Clematis will grow tall over time and the lower stems that are most evident to the average individual will stop blooming. This leaves all the beauty at the top. For this reason, pruning and rejuvenation is necessary at least every 3 years but ideally annually for best maintenance. Thinning lets in air to prevent pests, disease and fungus. It also allows sunlight to penetrate for more flowers and better photosynthesis. If you do not know your vine's group, you can chose to do nothing and the plant will likely thrive although flower production will lessen and be mostly at the top. You can also take a chance and cut back the plant to 5 inches. If it doesn't bloom well that season it is probably an early or midseason plant. A fountain of blooms would tend to indicate a late season plant. Either way, you may have to wait a season but it gives you a fresh start to train the vine and reduce the tangle of unproductive stems.