Practical invisibility isn't about being literally and utterly invisible; you can't go into a shop and throw the stock around and scream and curse and generally make the place seem haunted, or eavesdrop on people in the room with you, or kick a policeman in the 'nads and walk off laughing, or like that. Just as everyone above has said, that's physically impossible, and pretty sociopathic to boot. What becoming invisible is about, is making oneself harder to look at or to see, like vision just kind of slides over and off you without taking you in.
There are two methods of practical invisibility I've worked with. One of them is simple but only works locally: . There are so many of us around that there's probably one or two hanging out anywhere in the city at any time, and one just blends into the background.
The other one's a bit more complicated, and I'm having a hard time figuring out how to express it in writing without digressing all over the place and/or sounding like I'm full of shit. But basically one concentrates on not being seen as one moves along, using a concept or image with a quality of invisibility. The person who taught me about it recommended thinking of mashed potatoes, soft slippery mashed potatoes that the eyes just don't settle on; another might think of fog wisping silently along; I like to focus on brother Bat and sister Spider and how they can pass unseen through the night. Of course one would also do one's best to move about as quickly and ,quietly as feasible too, but what one's doing here is projecting their concept of invisibility outward so that the eye kind of...edits one out of its sight.
I can't explain how it works but I've used it many times when out on a scary street in a bad neighborhood after nightfall when I just did not want anybody to see me. So far, nobody has.