I have a problem with horizons. I cant get them straight. Luckily, Picasa has a really cool straightening tool. I also own a monopod, which is a great thing. They are incredibly convenient to carry around and a must for a telephoto lens.
Anyhow, I shoot from the hip alot (and keep a stiff upper lip), I take alot of photos without looking thru the viewfinder. Sometimes holding the cam close to the ground or above my head or with arms stretched out. Its cases like these where getting a straight horizon is difficult.
I developed a technique that I am calling the C.H.I.M.P. Grip (Catalano-Horizon-Improvement-Mono-Pod Grip). Its pretty simple, and the photo above probably explains it well enough. Essentially you hold your monopod horizontally, and you rest your camera on top of one end of it. The camera isnt mounted at all, its just resting there. It rests pretty steadily and comfortably. On my monopod I rest it right on the foam grip. You use your right hand to hold your camera against the monopod and you can than hold the monopod a little further down the pole with your left hand.
The C.H.I.M.P. Grip does two things: First it helps your horizons by simply allowing you to more easily see the horizontal positioning of your camera. Secondly, it dramatically stabilizes your camera from shake. You are really widening the stance of your grip thus the movements of your left hand are essentially geared down like a lever.
This is the monopod I use and recommend, very well made and very inexpensive: