While some people like to maintain a specific shape of the plant. Try never to prune them in a square shape or any rigid shape. As they look and do best when they are pruned to maintain a natural shape.
Cutting them in a box shape will result in spotty flowering and splotchy growth of the branches. When cutting azaleas to a desired shape, visualize a shape, and then just cut out the branches which are out of that shape.
To avoid frequent pruning, choose the right variety of azaleas. Satsuki Hybrids like gumpo grow only a feet tall, while some varieties of Southern Indica Hybrids like 'George Lindley Tabor' and 'Mrs. G. G. Gerbing' can grow till 10 feet. Hence, check the label on the bush, while buying the plant to see what variety it is.
When to Prune?
The best time for pruning azaleas is during the early spring, before the new growth on the plant starts. However, this will cut out that year's blooms. But, it does give the plant the full growing season to fill out, and time for the new growth to mature before the winter comes.
Another good time to prune this plant is when there are flowers on the plant, which means you will have to cut some of the flowers. You can also prune them just after the bloom, if you wish to have flowers on the plant. After they have bloomed, most azaleas tend to start growing the next year's flower buds soon.
Hence, pruning during mid summer cuts off the next year's bloom. Pruning in late summer is not a good idea. Because, late pruning runs the risk of tender growth being killed during the cold seasons. Also, cutting them in the fall should also be avoided.
How to Prune?
Start by cutting out the dead and diseased branches. Make sure you make a clean cut and on each cut apply diluted mouthwash to avoid spreading infection from the diseased branches. Then prune back the tall and gangly limbs which are shooting out from the top of the bush.
This helps to promote a very attractive yet compact shape. If you have older azaleas, then remove the tall branches in a span of several years, and not in one single pruning, as this will prevent shock to the plant.
Remove a few of the tallest branches, but take care to cut back to a side branch which is heading in the desired direction. When you are pruning these plants, don't worry about cutting it back to a connection branch as there will be new branches coming right from the areas where you made the cut.