How can one improve one’s letters of recommendation?

Query from a reader:

A good friend of mine who is not in a search committee but knows someone who is in a search committee told me my letters of recommendation are short and meaningless, and that this was the main reason why I wasn’t shortlisted (she has a good cv, but with such letters, there must be something fishy). How do I go about asking my referees in a tactful way to make a longer letter? What is supposed to be in a good letter of recommendation? How do I find new letter writers? (I will obviously still ask my thesis advisor, and he is kindly disposed towards me.)