I used fleece liners for a while, but eventually stopped using them to simplify my system. (hyenacart.com)
You can purchase a separate fleece liner for your baby carriers from this company to use during colder months. (abckidsinc.com)
The advantage of fleece liners is that they keep your baby's bottom drier than being directly against wet cotton, and poop tends to peel off of them. (hyenacart.com)