I have co-slept with all of my babies. I usually just have them inbetween DH & I to lay freely. Isla doesn't like to be in the middle though so she is on the outside (which does freak me out a bit). I do tend to lay with my arm up & around her head. Not actually touching her but just like a guard. But once she is asleep for the night I tend to just make sure the blanket/sheet is tucked under her so that if she did happen to roll towards the edge she will be on top of the edge of the sheet rather then actually falling out. but I wake the moment she moves so I am pretty relaxed that she wont fall out.

Another thing you could try is sleeping head to foot. I know its not the most ideal set up but I found it more comfy to sleep at the other end of the bed then DH, it just gave me & the baby more room.