Low flying is essentially a form of flying that only a few can really do well. The rules have been well stated here and should be adhered to.
I was trained in the Army in Auster 7's and 9's and I can tell you it is an exhilarating experience when you are a mere few feet above the ground. The real problem arises when you are unfamiliar with the area you are flying. The hidden dangers ALWAYS prevail. Such as man made structures that do not appear on any map, power lines that you suddenly find, that you have missed on a map, which you should NEVER attempt to go over, you will rarely ever make it.
Our army flying was carefully planned over well used areas of Salisbury Plain but unfamiliar areas are a danger.
One final word. Low speed may sound as though it is the safest way to low fly but this is a foolish notion. You should always fly at least cruise speed. That way you are more likely to get out of unexpected problem than to to stall your way into a problem, or worse, a crash.
Also, as Foxmoth quite rightly says. If you can low fly over the sea beware of the horizon and the sea merging. This is very dangerous situation. Just never go below 100'.