We have found that when it comes to embroidery less is more. Designs stitch out much cleaner when they are more simplistic. Stay away from a lot of outlining, shading and small detail. Just because a design looks cool on a computer screen doesn't mean that when creating it with thread it will look as cool on your embroidered garment. Try to stay away from very small lettering. You don't want people to have to get super close to you to read what your shirt says.
Try to choose garment colors that compliment your logo not match it. If your logo is blue and red you won't want your shirts to be blue and red or else your logo will blend right into the shirt.