A good explanation of a high plant can be any plant that is above shoulder level, and hanging plants fit into this description. At that height and beyond, it not only gets difficult to lift buckets or cans full of water, but it also puts you in an off-balanced position, which is the precursor for an accident waiting to happen. Taking down plants to water them is one solution, but for a less time-consuming and strenuous method, there are better ways.
If you have high hanging plants outdoors, consider getting a hose wand extension. They come in several lengths, some even telescope and go extra high, and all are available at your local hardware or lawn and garden shop. They are lightweight, literally screw onto the end of any garden hose, have an on-off valve, and even the shortest models easily reach 3 feet over your head. Pick one out with a length that is right for your application. One thing to note: They are best used in an outdoor situation where water runoff can just drip onto the ground instead of on your floor or carpeting.
Faucet wands act very much like hose wand extensions, except they use soft rubber connectors to slip on over your household kitchen or bathroom faucet. You can vary the pressure at the wand end by turning your faucet on high or low. They feature an on-off valve on the handle, and come in various-sized hose lengths that will reach virtually any place in your home from the nearest faucet.
A standard garden sprayer will work both indoors and outdoors for higher plants. They need to be filled with water, pumped up, and the inner pressure will send water as high as it needs to go. Each sprayer has a nozzle adjustment that will allow you to adjust from stream to mist and everything in between. Wand extensions can also be purchased that will allow you to reach the height of virtually any hanging plant.
For extremely high plants where wand or hose extensions may not be practical, you may want to bring the plant down to you. A simple pulley system with some decorated rope can be used to lower your plant to a comfortable watering height, then merely hoist it back up into position. A single pulley, found at any hardware store, hung on a hook high overhead is all you would really need. This would be an ideal solution for the plant that cascades and falls many feet below the pot it is planted in.
If hoses, wand extensions or pulleys won't work for you, try watering your plants with ice cubes. Ice cubes are lightweight and can just be dropped into any potted plant. Although you are limited to how high you can reach, two or three ice cubes per week may be all that you would need to add, depending on how large your plant is. It's an easy and effective way to get water up into any high hanging plant, without having the hassle of dealing with hoses, wands or sprayers.
All of these ideas will work well, but consider your own requirements and limitations before implementing any of them. Tailor a watering program to your style and choose the idea that works best for you.