Wearing darker colours make you look more slimmer while light colours have the opposite effect. So if you are bigger on the hips wear dark colours around there and lighter colours on the top.
If you are slightly on the bigger side or are short in height try to avoid wearing stripes or details on a garment going across your body. This will only make you look bigger. Instead try wearing stripes or detailed garments going down, this will have the effect of making you look slimmer and taller.
Try wearing bright colours or patterns to take the eye away from your problem areas, so such as if you are bigger on the bottom half, wear colour or pattern on the top.
Accessorise your outfits. This will take the attention away from your problem areas, such as a scarf around your neck or interesting jewellery.
Never wear clothes that are too small for you thinking it will make you look smaller because it won't , it will only make you look bigger. If you are a size in between then buy the larger size and have it adjusted.
If you don't own a pair of heeled shoes then buy one. They are a girl's best friend. Not only will they add a few inches to your height but will also have a slimming effect on you and making your overall appearance better. They will be one good investment you make.