I make sure the prefold is quite snug around my DDs legs--tighter than you might think necessary--because as soon as she moves or pees it loosens up, but making it snug it helped to contain those loose poops when she was younger. I used the thirsties covers and some bummis covers, both work, but I like the thirsties better--they seem to work a little better for my DD.
I also use the "angel wing" style folding. I make sure that when I fold in the sides that my DDs bottom is in the pocket area and that as I pull the folded part between her legs it stays narrow--then I spread the top a little and use a snappi to attach the top and bottom wings. The key, I have found, is to make sure that the diaper part that come across her upper (rear) thighs is snug and a sing layer--not folded or rolled (this sometimes just happens) and that it is snug. This seems to help with blow outs. Even doing this sometime, and I was always amazed she would still produce such quantities that it still leaked out, but not out of the covers most of the time.
Hope that helps.
(7 years) Mother to Lily
(13 months) 2