In the picture above, my fiance and I were set up at a popular rock climbing destination, camped alongside two other guys in hammocks who we didn't know. Take a guess which hammocks are ours. <img src="/ubbthreads/images/graemlins/wink.gif" alt="" />
The night was pretty cool, around 40 degrees or so. We both slept very comfortably, and the two guys to the left froze their butts off. Why? Because we were prepared. The wind was blowing in from the direction I'm facing while taking this picture, so if you look closely at the tarp configurations, I've got my tarp (middle of the picture) pitched low on the windward side to help block the air, and high on the other side (using hiking poles to hold the tarp up) to provide an awning for easy access and a place to hang out if it rains.
My fiance has her tarp pitched low to the hammock, with the foot-side tieouts crossing each other so the tarp is pulled closed on the foot end, again to block the wind. She's using a single hiking pole to hold one corner of the tarp up, again for easy access while keeping it close to the hammock and helping trap body heat.
We were both sleeping on Exped Downmat sleeping pads - I can not under emphasize how important that is! Most beginner hammockers don't do anything special for insulation underneath them, and they end up freezing. When you lay in a sleeping bag inside your hammock, you crush all the insulation underneath you, and with the hammock suspended in the air, you're basically completely exposed to the cold. You need to get some insulation under there!
The two guys to the left slept in a friend's tent the next night - they didn't have bottom insulation, and didn't have tarps to block the wind. We, on the other hand, got a great night of sleep, and were well rested for the climbing competition (even ended up winning some prizes!).
Lesson: learn to manage the weather to make your hammocking as enjoyable as possible.