this may have been discussed earlier in this thread or another, but width has alot to do with it too. i seem to get it more by laying more diagonal.
someone mentioned a trick that seems to work well for me. having your feet in the footbox, but then keeping or moving your torso closer to the shelf somewhat (about as far as you can without feeling like you're "tilted" to the left). (you'll still be on the same diagonal) i've found this makes a noticeable difference. i suppose i could make the hammock narrower which would force users into more of this position, but having the extra width is good for other reasons.
i've normally been very close to the zipper, but laying this new way, i'm several inches farther from the zipper and along with less calf pressure, it gives more headroom too.
as far as fabrics goes, the stronger the fabric is, the less it will stretch, and the more some people will notice calf pressure, the more the fabric stretches, the less noticeable it should be. of course, some people don't notice it at all no matter what hammock they have and others seem to notice it no matter what. those people might want to try moving their torso to the right like described above...some cushion under the knees works very well too of course.