Holly comes in many varieties, deciduous and evergreen. There are American species, oriental species and about 200 varieties of English holly. Evergreen hollies are pruned in early summer, which allows time for new growth to develop before the temperatures drop. Deciduous hollies are pruned in late winter. The manner in which you prune the holly bush will depend upon the variety of holly shrub that you have, and also whether you want a formal or informal shape to the shrub.
Cut away any dead, diseased or damaged wood immediately on both deciduous and evergreen holly shrubs (any time of the year). Make your cut at the point of the break, where the wood connects to the main stem, or at ground level. You will have to use your judgment as to where you cut. Your main consideration will be how it affects the appearance of the shrub. However, remove diseased branches completely to halt the spread of the disease.
Thin out any wayward or crossover branches on both deciduous (in late winter) and evergreen holly (in early summer) shrubs. The removal of these branches will maintain the natural form of your holly bush. Cut parallel to a main branch so that the thinning is not apparent.
Shape your deciduous or evergreen holly bush (if you desire a more formal shape), by shortening the ends of branches to a bud or leaf node (make your cut on an angle). You want to have the base of the holly just a little wider than the top of the bush. (Remember that timing is everything--deciduous holly shrubs get pruned in late winter and evergreen holly shrubs get pruned in early summer.)
Severely prune your deciduous holly bush in late winter to encourage new growth. Cut stems that are thicker than your thumb down to the ground. Remove old canes by cutting them down to ground level, and cut off any growth that appears gangly or that is out of the main body of the plant. Never remove more than a third of the bush.