Lar,
Most people want honest people to deal with and reasonable pricing on the products they offer. The word honest encompasses alot of things, including knowledge of the product/services you offer.
If you have employees, make sure they know the business/products and are not totally sales oriented. Yes sales are important, but the customer being satisfied after the purchase will bring them back.
Offer a variety of products that will do their job, and do not offer products that you do not believe in. Be willing to help those that need help, but do not be to quick to criticize. Learn when to talk / listen and when to get back to work.
As others have mentioned, make sure your tanks are clean and disease free. Be fair with you return priviledges, but remember to explain how to acclimate fish, corals and inverts. A how to sheet would not be out of line, and a water sample on returned saltwater/freshwater fish would not hurt either.
Be careful when special ordering items for customers, a deposit and your policy on these items should be made known up front.
If you use special lighting on your tanks, try to make the same lighting available in your store. Do not put in VHO, PC or MH lighting, and then do not sell it. The customer will wonder why your stuff always does better in the show tank than theirs. Offer test kits that you use, and do not be afraid to suggest others that will help. Have in stock some good books on what you offer, and don't be afraid to recommend them gaining knowledge before starting, especially in saltwater.
With livestock, offer items that will live in most tanks, and if you offer specialty items, make sure the consumer has a knowledge of how to care for it. You can not be everything to everyone, and you will learn how to "read" people over time. Set your rules and stand by them.
Be open atleast one evening, and remember to take time for your family and your own needs. I wish you the best of luck in you venture. I wanted to do that about 32 years ago, but could not get backing, I was only 16 at the time. Now I would like to be a coral farmer, and hopefully that will work out.
Again, I wish you the best of luck, and don't forget to keep in touch with us in the forum.