Normally we have to take our attention away from the breathing and bring it to the body. Feeling the body sitting, the touch points, feeling the touch of the buttocks, the hands, also trying to note it, feeling it, holding it, noting it and then righting ourselves. Another thing is to put the hand down like the earth-touching posture of the Buddha with the fingers pressing the ground. Then as you fall asleep and go forward, the pressure on the forefinger tends to wake you up. This can help a great deal.
Another thing is to be careful about what meditation subject you do when you are in the sleepy state. Watching the breathing is not appropriate when you are in a sleepy state. So try to have a reflection that is more energy inducing. For example, confidence-inspiring objects are energy inducing. As well, reflection on death can often get us out of that sleepy mode. These types of reflections can be very helpful. If we are actually just flowing with the breathing, it's very easy to go into a sleepy state. So it might be especially helpful to do a joy-type meditation, sympathetic joy helps to brighten the mind, or how fortunate we are and how precious our opportunity is and not just how fortunate we are to go to sleep! This may help us to brighten the mind during the times when we're sleepy.
Also, sometimes there are periods of the day that tend to be sleepy for certain types of people. So we may have to know when these periods are and know what type of meditation is good to do during that period. If it's after a walking meditation, it may be helpful to pay extra special attention to your walking just beforehand and actually not do any slow walking. Instead do more brisk walking to get the energy going. Sometimes because we're walking we don't feel so sleepy, and so if we understand that the next period is normally a sleepy one, we can bring more energy into the body with brisk walking.
If you have a choice to stand and you are sitting down, then stand up. It's very difficult to fall asleep when you are standing because you know how dangerous it is. So that extra body awareness tends to wake you up. If you are standing, then be aware of the body and the feet more, even if it's for the whole period. So these extra hints can be very helpful.