This question was sent to me by a single mom who wants to do the right thing but can't afford to over tip <img src="/images/graemlins/confused.gif" alt="" /> My answer was at least 10%. I have decided post to my answer in the forum to see if someone has a different suggestion.
I know that the buffet server's do not bring your meal, however, most do bring water and beverages to you, they remove used plates from your table, etc. Also, 10% seems perfect for buffet since 15 to 20% is suggested for a full service restaurant.
When tipping make sure that you are tipping on does not include tax when calculating the amount. Also if there are 5 or more in your party, make sure that the tip has not already been added in, some places do that even when it is a self service or buffet restaurant.