Something that helps for me is to move my head regularly (not just turning, but a few inches to one side or the other as well). This changes the relation of what's out there to the background, previously explained as why converging aircraft are hard to spot.
My glider canopy has a hoop between me and the nose, so I'm conscious that there's a blind area. I find that moving my head to look "past" the hoop will sometimes reveal something in the clear area, just because I'm looking at it at a slighter different angle.
Does anyone else think this helps?