From the "Lessons Learned" section of my journal:

The real problem is almost certainly not what the customer says it is.

Also:

Sullivan's Rules for Getting and Keeping Clients:

• Listen to your clients so that you understand their business needs
• Match leading-edge technology to the client's goals
• Explain in business terms how your solution will help the client meet goals
• Deliver on time and within budget
• Use a framework that helps ensure quality control
• Network with associates who can bring value to your projects
• Use your success stories to sell yourself to clients
• Keep your technical skills up to date through training
• Develop good verbal and written communication skills
• Share knowledge of IT trends with your clients

- Romeo, Jim: "The Fine Art of Getting and Keeping Clients." Contract Professional, Vol. 4, No. 7, March 2000, pp. 28

Source unknown:

Know your priorities. A computer that crashes constantly isn't as important as part of a network that has suddenly gone down.

Murphy's Law states if anything can go wrong, it will. Always be prepared for anything that might happen throughout the day. Keep a backup plan for everything.

Know how to deal with everything on your network. It's good advice to know how to do everything, from running the server to adding new hardware in a machine.

Use a team effort whenever possible. Things get done faster when you team up to work with your fellow IT peers.

Know your users. This helps you understand their wants and needs, and allows you to provide assistance to them much more easily.

Give extreme timeframes. If someone needs something done, tell him or her it will be done in a week, and hand it to them 3 days later.

From Tech Republic:

Develop and test a backup plan.

Schedule hardware upgrades during a known good environment.

Communication is key.

Walk away from a problem to take a deep breath, then return to the troubleshooting basics. We were so intent on fixing the problem that we neglected to first identify the problem.

Never anticipate that you can have a smooth day, get off on time, and enjoy hours of play online with Rogue Spear.

From: Just another smooth-running day in IT . . .NOT! (original link - still works, to my surprise)