What do you use? Under quilt??? Top quilt???? Sleeping bag??? I am buying a hammock, probably a Clark Jungle TX-250. I'm not sure how to keep my but warm.

You should join the private "Hammock" thread here and ask there. To join just look at the bottom of the main forum page and click on the private groups you'd like to join.

Personally I use my down mummy bag as a top quilt and use a 1/8" closed cell foam pad (extra wide) in the sleeve under me. I have a Warbonnet blackbird and it has two layers of fabic under you where you can slip a pad. A very nice feature. But I do look forward to when I get that 20 degree incubator down underquilt.

Here's a link to the "BWCA Hammocking" private group forum: BWCA Hammocking

I have a Clark as well as a Hennessy. I use an underquilt (a Nest on the Clark). I then bring a mummy bag and unzip is 3/4 way down and use it as a top quilt with the zipped bottom a foot box. We've done this down into the 30's and kept really warm. On the Hennessy i have a top cover I can pull down. when I do that, however, it actually gets too warm (again in the 30's).

Reading input on the hammock group will add detail, but in general compression reduces insulation and being in the air the butt will get cold without some form of insulation. Pads with or without added material such as tyvex or other to assure full under coverage (when an arm or leg falls off the pad it gets cold)are in competition with underquilts. Underquilts tend to win out as favored, but can cost more. I think most would agree until it dips into the 30's a pad will be okay.

I have a Blackbird and tried a pad but upgraded to a Hammock Gear 20 deg Incubator under quilt shortly after I got the hammock. I also use Dutch under quilt hooks and its real easy after I got it all dialed in. There are clips on the Incubator and after I got everything how I wanted, gave a couple stitches into the hammock to hold it there.

I used a down mummy bag for a year and then did another upgrade to a Hammock Gear Burrow 20 deg top quilt with 2 oz of over stuff. I got this during the Black Friday sale. Have three nights in it so far and I like it as well.