Some rhododendrons get so leggy you might want to chop them out. If you are in a hurry, this might be the best thing to do. It takes time to correct the problem. But if you have patience, you can often restore a plant that has gotten too gangly.
The best solution is often cutting off most of the plant, especially if there is some fresh growth near the base. These thick shoots and larger than normal leaves can produce a more compact new plant more quickly than you might think, similar to the case of unintentional pruning described in Damage Recovery. All the energy of the plant is focused on growing from the bottom, rather than sending nutrients and water through long, sinewy stems. Don't cut below any sign of leaves or leaf buds, or recovery might be too much for the plant. Some, such as r. thomsoni, don't seem to sprout low on the plant, and will probably die if chopped off.
Another solution is to tie up the lanky branches. The picture on the left is of the plant above, ten minutes later. (So much for patience.) This doesn't really solve the problem, since the rhododendron will still be leggy, even if more confined. It is still a good idea to do some pruning to force more sprouting and get the plant more dense.
Better than fixing leggy rhododendrons is preventing them from getting that way in the first place. First, make sure they have enough light. Though most need open shade, too much will make them lanky. Second, prune the tips before radical surgery is required. Finally, make sure each plant, especially the larger ones, have enough room.
(I'll include a picture of this rhododendron's full recovery in a few years.)