Here are some simple rules to follow:
1) Keep it short:
A short name is easier for people to remember and write down.
2) Keep it generic:
A generic name initially gets indexed better by search engines and is easier for people to remember. A domain name that describes what you sell can be very valuable.
3) Register all variations:
If you are operating in multiple markets or your name has ambiguous spelling, register all variations that customers might try (eg. .nz and .com, two-t's and one t in the name etc). For a small additional cost you are protecting yourself from losing customers long into the future.
These three rules will help ensure your business has the best chance of success!