I am by no means an authority on anything to do with the internet.  I will, however, offer a few suggestions of what I have seen work.  For more detailed information, please, either google it or you can check out this site.

1. Get a '.com' - it is what people type when they don't know otherwise.

2. Either match your business name exactly, or have useful keywords added (keyword=anything people might search for to find your site).

3. Keep it short and easy to remember. Sometimes this is easier said than done while following rules 1 & 2.  If your stuck, follow 1 & 2.
   

4. Avoid hyphens and usually numbers.  People forget where the hyphen went and forget whether you spell out the number or use the actual number.

These are what I deem to be the most important rules to remember.  If you are talking it over with your friends and you have to repeat yourself a couple times, its probably not a good name.  If you need help pick out your next domain name please feel free to contact me and I will offer any suggestions I can think of.