Determine a scheme for your bathroom. Stick to something easy to understand and envision. If you can not envision fixtures that match the theme, try something else. For instance, a classic, or modern theme it very easy to conceptualize.
Look through faucet catalogs and pick out faucets for the shower and bathroom sink that match this theme. Circle them with a pen so you can come back to them later. Do this before you get into the home improvement store as it makes the selection process easier. You will also be at home so you can try to visualize how the faucets look in your bathroom.
Stick to faucets in the same series from the same manufacturer, or faucets of the same design type from the same manufacturer. For instance, Delta has series faucets like the 17, 18 and 19 series. Choose faucets from a series to make a better match.
Pick faucets that have the same finish. The finish is listed in the description in the manufacturer's catalog. Chrome finishes are the most popular, however nickel, oiled-rubbed bronze and brushed steel are also available. Matching faucets should always have the same finish.
Stay with designer styles or simple styles. Do not have one designer faucet and one builder-grade faucet. The designer faucet will stick out and the two will not match.
Match the flow rates of the faucets before you finalize you decision. Most faucets in the same series from the same manufacturer have similar flow rates, but check to make sure. This information is listed in the catalog under specifications.