Compose with your main subject at or near the center of the image and often very near the lens.
Place the horizon in the middle or just above or below the center line to keep it more or less straight.
Compose to frame your central subject within the curves of any objects along the edge of the image.
Get down low. This and other unusual vantage points conform to and elevate the fisheye look.
Focus close. The best fisheyes let you get an inch or two from the subject. Nearby objects loom large while those in the background shrink, creating an almost three-dimensional sense of depth.
Try slower shutter speeds. The 180-degree field of view is relatively forgiving of slight camera movement.
Use software. You can easily level a horizon line by selecting the image circle and turning it with, say, Adobe Photoshop's Free Transform tool. Or, if the outer circumference of your image is soft or too distorted, you can make a circular crop into the image with the Elliptical Marquee. Even better? With stitching software, you can easily combine several 180-degree images into a 360-degree, full-scene panorama.