Unless you are willing to chop of your legs at the knees or remain seated the rest of your life, nothing is going to change your height. You can, however, make yourself shorter with some strategic tips. While you will still physically be the same height, you will appear shorter to others, which works to achieve your goal. You can make yourself shorter with a few feasible tips that don't include lopping off limbs.
Wear flats. Even one inch heels will add, well, one inch to your height. Pick the flattest shoes possible, but don't skimp on style, comfort or support. No, you don't have to spend the rest of your life in flip flops or ballerina slippers, but find some flat shoes that are still cool and offer proper footbed support. They are out there so go shopping.
Dress in visually shortening clothing. Steer clear of long, flowing dresses, vertical stripes and monochromatic outfits, all of which automatically add the illusion of height. Go with cropped jackets, shorter skirts, horizontal stripes, a bold colored top with dark pants, capris, thick belts and clunky (flat) shoes.
Slouch. This step is not recommended at all, since it can make your spine twisted and lead to problems later in life from not standing up straight. It also make your body appear saggy and reeks of someone who is very unsure of herself. But in a pinch, it will also make you appear shorter.
Stand strategically. When standing around chatting with others, you can position yourself so you are at a lower level than them, at least height-wise. Step off the sidewalk everyone else is standing on; stand at the bottom of the steps or lower down on a ramp; lean on the bar, pillar or other nearby supportive item.
Surround yourself with tall people. Get yourself some very tall friends, frequent a lot of basketball games or find other venues where tall people abound. Even 5 feet 11 inches can look miniscule next to others who stretch further towards the sky.
Another option is to embrace your height as a beautiful gift. Taller people can eat more, reach high shelving units and often intimidate people to get out of the way. Wonder Woman was 5 feet 11 inches tall and she proud of it, too.