Over the flats (both flannel and Terry) I use Snibs and a those fluffy covers... can't remember their name... BUT you have to use about 1 size larger than your baby... don't buy the smallest size. They actually get better as they get older... at first they are a bit tight. Ahhh wish I could remember their name... maybe "Fluffies". They are still available... essentially undie shaped covers made from very fluffy nylon. Sometimes during the day you can get away without using a Snib... but they are essential at night. I've also been known to use training pants as covers.

From my experience terry flats are better at night (more absorbant) and flannel ones better during the day (not as bulky, better for mobility).