The rule is just “K.I.S.S - Keep It Simple and Short”
Some tips for the creation of domain names:
a) They should not be too long to remember, for if a person can not remember it, he/she might not come back.
b) They should be short, simple and nice sounding names, so that the person can easily remember or recollect the sites’ information like the famous online names like Yahoo!, Google, MSN etc
d) However, names should be short, avoid long acronyms. Initials are a strict no-no.
e) The domain name should reflect the product or service which your business is offering as it helps the visitors to remember. It can be like this site, which had chanced upon,
http://www.stopsnoringadvice.com, which gives some advice to stop snoring (the advice given does help).