It all depends on what type of working/spell work that I am doing if I cast a circle or not. But for me when I do I like to cast a circle with a circle invocation and visualization.
I will stand in the middle of my working space and say something like:
"I conjure thee O circle of power, to be a boundary between the world of the mundane and the world of the spirit. Be a barrier and protect me from all things that may wish me harm. Be a container for the energies I raise within. As above, So below. The circle is sealed!"
While I say an innovation like this (it changes from time to time) I visualize energy coming up through me from the earth and out of my body creating a sphere of light that encompasses my working area.