wear all black, always. Or all dark grey. If the colors are neutral and monochromatic, you'll automatically look sharp, even if you're just wearing a baggy sweater and jeans. Darker colors are great because stains don't show and your face will stand out from your clothes.
Invest in one good-quality bag, one pair of shoes, and one belt that all have matching hardware. For example, the metal parts should all be gold or all be silver.
I do this and get loads of compliments on my outfits.