Stay warm! Underquilt is necessary for hammock camping – if you do not choose to slide around on a sleeping pad of course! Synthetic underblankets – like Snugpak underquilt – are large, heavy, but cheap. Down underblankets – Bushmen GLOW or Cedar Ridge Underquilt for example, are light, warm, compressible, but of course slightly higher price range.
Topquilt or hammock sleeping bag is also comfortable, but of course you can sleep in your hammock with a regular sleeping bag.