I've never tried a quilt, as recommended by andrewclaus, but I've heard good things about them.

For the past few years I've been using a 40°F down-filled Big Agnes bag. They're notable for having insulation only on the top. The bottom is uninsulated and has a sleeve for an inflatable air mattress. (I use the Big Agnes Air Core.)

I also carry a Sea to Summit "Reactor Thermolite Liner." I like a liner for several reasons: 1) Adds warmth when it gets too cold for my 40°F bag; 2) Good by itself when it's too warm for my 40°F bag; and 3) Helps to keep the sleeping bag clean, and the liner can be readily laundered.