My rules...
1. Always use the lowest ISO you can... 100 generally being the best quality.
2. Pick an aperture to get the depth of field that you want, and that is compatible with the quality of the lens (some lenses aren't sharp at the largest aperture which is the smallest number). If you want a narrow depth of field to blur the background, pick a low number... if you need to photograph a large object, group, landscape, etc., pick a larger number.
3. Finally, make sure your shutter speed is fast enough to avoid camera shake and stop action if the subject is moving. Something around 1/60 is usually OK with a normal lens and non-moving subject. If you are shooting sports, you probably need more like 1/400.
4. If you can't get the combination of aperture and shutter speed that you want for your creative purposes, then start increasing ISO as needed. Know the capabilities of your camera so that you can make proper trade-offs as ISO increases. Example - Would you rather introduce a little blur, or increase noise?
