What can I do to help avoid laxity while meditating lying down? My upper back muscles are not strong enough yet to support seated meditation, so I use supine/savanana posture sometimes. My mind falls into short dream states, like hypnagogia, very easily in this posture, however.

Any tips for overcoming this laxity while doing supine meditation? I am only doing samatha meditation these days.
The Meditation on Falling to Sleep:

We can be on mindful guard for the arising of the moment we  start to fall asleep. This mindfulness should stop the falling asleep and can give an  initial boost in the meditation faculties like effort and concentration so we  won't fall asleep as much and we can then better practice whatever we want with those improved faculties and wakefulness.

We can know when we are starting to fall asleep by finding and tuning into were  the bodily or mental experiences are that we know always happen right before we start to fall asleep. We then  can easily catch when we will  start to fall asleep.