What symbols does your area use for marking points on the floor? Circular cross hair for a 1 Ton motor? Triangle for 1/2 ton? Square for 2 ton? Or do you use those shapes for differentiating departments like Audio, Video, and Lighting?
I have always suggested patterns that can't logically be made from each other for different things, try to alternate diagonal lines or use circles and squares as polar opposites. The reasoning behind this is that if the floor gets all scuffed up and somehow some tape comes loose, you should always be able to tell what the symbol should be. Also, having symbols that mirror on each axis is also good because if up to 3/4 of the tape is removed, the original symbol should still be obvious. The letters X, O and the symbol # are examples of this, where the letters B, Q, Y, etc are not. I know this really isn't that big of a deal, but I just try to think of every possibility.
That's pretty much exactly what we used to use (although the first symbol meaning both a choke with a down leg and the more standard basket with a down leg is a little questionable). There are some really big shows out there that use different symbols for different departments (square - set, circle - lighting, triangle - audio, etc...), but I've only run into that a few times.