Sitting meditation should be preceded by walking meditation. If you have an hour for meditation, do at least 20 mins of walking meditation first. If you know that you are going to feel sleepy, then do 40 mins of walking, and sit for only 20 minutes.
A good sitting posture is also a great help to defeat sleepiness. Ajahn Chah recommended sitting on the edge of a deep well.
At least, try to sit straight, without any support from a wall. Use a thick cusion if necessary, or a Zen kneeling stool.