1. Show off a little foot. Extend the line of your leg by keeping the top of your foot exposed. Skip the Mary-Janes, the sneakers and the sandals with the ankle straps and instead slip into flats, pumps or even a basic flip flop.
Both of these images are of LOFT cropped pants with the same inseam measurements, but the shoes make a big difference. The image on the left is paired with shoes that expose the top of the foot which creates a singular line from the bottom of the pant to the toes. Even without wearing a heel, this will create a longer looking leg.
The length in the image on the right is completely dependent on the sky-high wedges that the model is wearing. The ankle strap disrupts the eye and starkly delineates where the leg ends and the foot begins. If, like most of us, you do not have the proportions of these models, you'll need to be aware of the lines you are creating with your clothes. Generally the aim is for a long and uninterrupted line so as to create length instead of width.
3. While many retailers are moving more and more towards slim and skinny fits, remember that you will be wearing these pants in hot (and often humid) weather. Although your pants should be creating a straight line, ensure that there is enough width to allow for air flow and movement. I highly recommend avoiding pleated or cuffed crops across the board since they, more often than not, add extra width. If you generally wear wide-leg pants, your cropped pants should feel slimmer than you are used to. If your cropped pants are wide at the bottom, your exposed leg will look even smaller by comparison and will create a top-heavy proportion.
The next time you are trying on cropped pants (either in your closet or at the store), test them out with different shoes and try cuffing them so you can see the difference those things make in your own proportions. The more you see what works and what doesn't work on your own body, the better you will be able to choose great pieces.