1. Be different - but in a good way. You may have heard the saying that there's no such thing as bad publicity. Yes, there is - just ask Rupert Murdoch. Make sure that you stand out for the right reasons. Keep the WIIFM principle, otherwise known as what's in it for me (from your customers' perspective) at the centre of anything you do.
2. Do something great - either for a client, for your staff or for the world in general and let people know about it. Everyone likes to think that business decisions are based on rationale and logic. They're not.
Everyone - yes everyone, buys on emotion and then backs it up with logic. So do something authentic for you and your organisation that makes someone somewhere feel better about their day - again keeping the WIIFM principle top of mind. You might also want to have some logical reasons for doing business with you handy too. Customers like logic to back up their emotional purchase decision (yep - even the business to business buyers).
3. Keep it consistent - Make sure that every part of your business (and yes, that's more than just your marketing materials) says the same thing. If you pride yourself on service, but it takes your receptionist 10 rings to answer, there's an incongruity there. That incongruity makes people feel uncomfortable. If it happens enough, your customers will have a level of discomfort that turns them and their business away.
4. Do what you say you'll do - a lot of people don't. When something goes wrong (maybe it's outside your control) that's when keeping on keeping on really counts. After you finish swearing, downing a scotch, panicking, etc - sort the problem and get on with it. There's no need to share the drama with your client - you'll know you've gone above and beyond - bask in the inner glow of a job well done - problems and all.
5. Give good love - client love that is. Send thank you notes - yes lots of people say it and still so very few do it. Send something nice with your bills to your favourite clients. Send things for Christmas, anniversaries, birthdays, big wins, etc that you know they'll like. I gave a client of mine who was very into beer, a beer tasting box - he loved it. I know I could have got him a bottle of wine, but it would have gone either in the office kitchen or to his home relatively unappreciated. The trick to this is to pay attention when they speak.