Stand with your feet shoulder-width apart. Bend you knees slightly while keeping your back straight. This helps open up your airway, gives you greater control over your diaphragm and helps you open your throat for singing.
Breathe deeply using your belly instead of your chest. Many people tend to breathe more shallow as they get older, taking breaths in and out through the chest. Deeper breathing requires use of the stomach and diaphragm -- this will give you greater control over the palette for releasing the throat.
Take a sharp, deep breath without sound. This will force the throat open temporarily. The more you do this, the more you will recognize the sensation, which helps you gain control over those muscles.
Force yourself to yawn. This also opens the palette for a longer period of time and further develops the muscle so you can duplicate the release while singing, without needing to yawn. This will also help you hit higher notes.