1) Flat subjects don't require deep depth of field. If you're photographing a flat subject, position yourself so that the plane of your camera is parallel to the plane of your subject and use an aperture of around f/8 or f/11.
2) Make sure your center of interest is in sharp focus, Don't frustrate the eye by leading it to parts that are of particular interest and then hiding those areas in blur.
3) Bright days work best for macro subjects. Ideally, you'll have an overcast—but not too overcast—sky to soften the light. The bright light will allow you to get greater depth of field without resorting to a higher ISO or slower shutter speed.