Invisibility to human sight is not difficult. Our eyes are not very perceptive and there are a plethora of examples in the animal kingdom of more advanced eyesight. We cannot see radiation, microwaves, gamma rays and all kinds of other thigns that can kill us.
Military engineers have already developed suits which can make vehicles and humans invisible to the human eye. Basically, a suit full of fiberoptics takes the image behind the wearer and projects that image to his/her front; which looks something similar to the creature in the Predator movies.
You may also want to look up the Philadelphia Experiment, which was rumored to have made a navy ship invisible...and a whole bunch of other crazy stuff.