If you are trying to improve your conscious projections, lying on your back with hands clasped over your Solar Plexus region would probably be the most effective.

But if you are just wanting to enhance your sleep "projections" then it doesn't matter if you are lying on your back or side, just try and sleep with your arms crossed/folded over your chest area if you can.

Another thing you may want to experiment with, if this is for sleep only, is aligning your body with the magnetic compass directions.

So try sleeping in turn for a few days with head to the North, head to the South, head to the East, head to the West. You should notice that the quality of your sleep is noticeably better/worse in certain directions.

And another thing to consider is there are certain phases of the moon when you'll find your night-time psychic activities are enhanced. You'll have to experiment for yourself (by keeping a journal of your experiences over a number of months) but I think usually that the period leading up to the Full Moon is especially powerful for many people.

And, finally, your intent for what you want to do with your sleep-time is probably far more powerful than any of the above. If you fall asleep regularly with a certain intent in mind, it will lead to results.