Normally we use 'Yet'at the end of a clause."Your dinner is not ready yet" but when we
use negative auxiliary like not(have/has),"Yet"comes after it in order to add more
emphasis in other words,using stronger language and emphasize on the subject (that is
currently in progress).like you have mentioned above "Your dinner is not ready yet".
to avoid a lack of clarity I wanna give an example,in a movie someone(a young man)
wanted to be a king,when his father found out that he is not worthy to be a king he
said (with a loud voice and strong language)"you are not yet ready to be a king".
it depends on the situation that you are in,in the example above he(the son)insisted to
be a king but his father strongly opposed him.he could say "you not ready to be in king
instead he said "you are not yet ready to be a king".
You can also check http://www.usingenglish.com/forum/threads/104300-use-of-yet-in-a-sentence
as Yenigun has mentioned.