Thanks to Susanne for this Frugal Baby Tip!
Cut up an old wool sweater to make a bum sweater. I just do a double layer through the middle and they're just as good as an aristocrats and only cost as much as a thrift store sweater and some elbow grease (can't knit for the life of me, but I do sew).
If the diaper is totally dripping inside, my bed is still dry, but the Bummis don't do so well if you can wring the diaper out, if you know what I mean. It helps with absorbency too.