When it comes to shrubs, in the long run, tough-love pruning is better for a plant's health than letting it grow unchecked or simply cutting back small branches here and there.
Rhododendrons require vigorous growth at the bottom of the plant. If the bush is leggy and thick at the top, light can't reach the bottom areas, and growth is thwarted.
To solve this, cut the bush back severely, all the way to the area of divided growth nearest the bottom of the plant. A handsaw may be necessary to prune thick bushes.
Severe pruning is a long-term commitment, but in two to three years, a rhododendron that has been cut back in that fashion will be a whole new plant.
February, March, and April are the best times to prune in the Northeast. In warmer zones, stick to the late winter and early spring months, before the first bloom. New research has shown that there's no need to paint over the areas you've cut away.