Thanks to Karyn for this Frugal Baby Tip!
What I do is use cheap terrycloth washcloths, cut in half so they measure about 5.5 in by 12 in, and zig zag stitched on the raw edges, as diaper liners. These do two things: add a layer of absorbency, and protect the diaper from poop stains (not completely, but they help a lot).
For overnite, I also add a polyester mesh stay-dry liner next to my son's skin. For daytime, when I think my son may be about due to have a poop, I use a Kooshies flushable liner just in case (overtop of the terrycloth liners).
Just a note on Kooshies flushable liners:
Advantages-- it's great to be able to just lift out the pooped-on liner and drop it in the toilet and flush; no toilet dunking.
Disadvantages-- it only works if your baby has solid, formed bowel movements (not helpful for a breastfed baby's runny poops); also I don't really like the idea of putting that scratchy paper next to my baby's skin. It kind of negates the use of a cloth diaper, IMO.