Riding your motorcycle is fun, but when you ride for longer periods of time, your body will start protesting. At least, that is the case for many bikers. Apart from the famous monkey butt, one of the main areas of pain is your back.
If you think about it, or analyse it, your back will take all the strain of your riding posture. A lot depends on several factors; your body measurements, your motorcycle type and some parts of your motorcycle.
TYPES OF MOTORCYCLES:
Basically there are three types of bikes, Standard, Sports and Cruiser. Each has a body position, feet position and hands position. These three parts will form a portion of your riding comfort.
Standard

Standard Motorcycle:
In the Standard motorcycle type (typically dual sports, touring bikes), your body is by default straight up, feet’s are directly below you and your hands straight. This is by far the best position for riding a motorcycle for longer distances.
Cruiser

Cruiser Type:
The second best motorcycle type is Cruiser. Like the Standard type, your body is straight, your hands are straight (unless you are riding an extreme ape handlebar) and your feet are slightly ahead of you. Your legs will “hold” your body less than the Standard type, but your body will remain reasonable straight.
Sports

Sports Motorcycle:
The Sports type requires your body to lean forward, and at higher speeds your torso will be required to fight a strong wind, while your hands are lower and your feet are behind you. In other words, there is a lot of stress on your body, one of the reasons you cannot really go that far on a sports motorcycle.
Your Body
Looking at the above motorcycle types, your body measurements will have a big influence on your back. Obviously your body mass (i.e. obesity) will play an enormous factor, but then it will in other aspects of your life.
If you are above average height, you will stoop, hunching your back and thereby creating pain. Raising your handlebars will alleviate that issue. If you are smaller than average, the problem will be different, but the area that will cause your back ache will be your feet. Obviously lower your handlebars (if possible) will help, but few bikes can do that. If your feet reach the ground properly (if they don’t, change your motorcycle), then see if you can raise your foot pegs.
The objective is to straighten your back and keep it straight.
Motorcycle Parts
There are three parts to your motorcycle that can be adapted to make it easier on your back; handlebars, seat and foot pegs.
 
Handlebars
A motorcycle’s handlebar is made for an average height of the biker. It is obvious that a big percentage of bikers are not the right height, either too small or too big. To make your life more comfortable, and less back aches and hand/finger numbness, you can change the handlebar on your bike for something that fits better. Taller, shorter, wider, etc. When you look at the handlebar make sure it fits your body measurements.
Seat
Usually the stock seat of a motorcycle is of average quality, and changing the seat for something more comfortable and more adapted towards your body measurements will do wonders towards riding longer distances.
Just adding a Airhawk can make all the difference.
Foot Pegs
Changing the position of your foot pegs will change your body posture. Many bikes allow you to lower or raise foot pegs, and if you buy aftermarket pegs, you can get something that suits your body better.
An alternative to standard foot pegs, depending on your bike are floor boards.
Summary
As you can see, your back ache comes from different areas, and you can help yourself by changing some part of your motorcycle.