If mildew is noticed in its early stages it can be easily controlled by many different fungicides readily available. Your local garden center or nursery will be happy to make an appropriate recommendation here. Earwigs and slugs are the most common pest problems clematis have. Slugs are the most troublesome. As they will always be with us, it's a matter of control not elimination.
Slugs prefer to attack young shoots, thus slug bait early in the spring gives the best results.
Earwigs usually attack in mid summer and can turn the bloom of a late flowering clematis into a lace work over night. There are many good pesticides and traps available to control earwigs.