For your monopod/tripod shooting with a long lens I reckon a gimbal head could be a major improvement. In this type of head, the camera/lens hangs below the pivot point and tends to self stabilise whereas on a conventional head, it is perched on top of the pivot point and inherently unstable - it always tries to fall over as soon you loosen the head to allow movement.
In the D800, Nikon has given us one of the best Auto-ISO functions available. You can select limits beyond which the camera will vary ISO to maintain the selected shutter speed or aperture. You can select a variable minimum shutter speed that responds to changes in focal length when using a zoom or changing lenses and you can even adjust the sensitivity of that response to focal length changes. generally speaking this camera seems to require stricter adherence to the inverse rule (SS = 1/FL) than bodies with lesser resolution, in spite of VR. Some say we should use even higher shutter speeds - I have seen SS = 1/2FL proposed in a few places
For handholding I tend to brace my body or the camera in several ways depending on the situation and posture: - brace the tip of my right thumb against my forehead or eyebrow by extending it (I shoot left eye to viewfinder, if you use right eye then try extending thumb to temple - but not as effective), - left hand under the lens, not holding the camera body, - lean against a pole or wall, or even brace the camera against it - pull elbows in against my ribs to provide stiffer support to the camera - sit on the ground and brace elbows on knees, - one elbow on one knee and one elbow against the ribs when kneeling
So far I am seeing that the D800 does respond to this extra care.