How can you improve your stretch, strength, and endurance all at once? Improve your posture. Just what your parents told you, stand up straight! Actually, it is not so simple. When you stand upright, with good posture, you are using the minimal muscle tension possible to maintain that position. If you stand as straight as you can, your muscles are less tense, when you use them you will have greater contractile force, and you will be able to use them more than if they were tight and tired (IE increased endurance).
In fact, many of the issues of tight muscles that people encounter when they attempt to stretch, are due to bad postural habits. For example, lets say you sit slouched at the computer, neck muscles tense to keep your face from hitting the screen. When you go to stretch your neck, it is going to be much much tighter than if you sat at your computer upright, and did not lean forward. As a rule of thumb, if I would look at you from the side, your ear lobe should be centered over your shoulder, which should be centered over your hip, which, if you are standing, should be centered over your knee, and the weight should fall into your feet with a line slightly ahead of your ankle. This is a plumb line from your ear lobe to the floor.
So, if you work on your posture, you will be astounded to see how much your stretch will improve as a result of that. Try this exercise: stand with your back to a wall, all parts of your back touching the wall (hips, low back, mid back, neck), and bend your knees slightly. Begin to feel what a flat back feels like. Now, if you have shortened neck muscles, you may not be able to do that part of the exercise, but just beginning to comprehend what a straight back is in a standing position will help you a lot. Of course, when you move, the curves in your back will return. But working on a straight back against the wall can stretch the ligament that runs down your spine and keeps you from being able to perform certain stretches that require full body flexion.
Check out my DVD if you wish to learn more about the muscles that hinder posture, with how to stretch them out!