Constipation is annoying, especially when sitting on the toilet and bower movement is not promising. Though the best way to solve constipation is to diminish the reason for it, here is a simple way that may help.
The key is being aware of the movement of your diaphragm when breathing. When breathing in, try to push the diaphragm down, thus naturally push the belly out. When breath in, let go the diaphragm, so it would go back up again, then breath in, diaphragm being pushed down, belly out, keep doing this for a few minutes. This will stimulate the bower movement, therefore reduce the constipation.
This is actually one of the most popular breathing methods, and it can help much more than just constipation. However, in many cases, the focus point is on the belly movement instead of diaphragm, which is taking the result as a method.
The best way to practice it is lying on one side of your body in bed, make yourself feel comfortable and relaxed, also make sure there is enough room for belly movements, then practice it for several minutes. When you get used to it, you can do it in any gesture, such as when sitting on the toilet. In either way, it will help.