Use a color wheel to find complimentary colors.
Every color has a complimentary color. It's just whatever is on the opposite side of the color wheel. True complimentary colors are the same distance from the center of the wheel, but it can be very striking to combine a color from the center, like a pale yellow green, with its complimentary color from the outside edge, like a deep mauve.
Let a print be your guide.
Find a print from a designer you love (or even something in your own wardrobe) and break it down into its different colors; then make an outfit. If you want to take it to the next level, rank the colors in the print from most used to least used and wear larger items like dresses, coats, or sweaters in the most-used colors and smaller items and accessories in the least-used colors.