When you say "seemingly diligent preparation" do you mean that you studied daily?
Did you make flashcards, or at least use someone else's flashcards to study from on a regular basis?
Did you constantly update your flashcards with newly acquired knowledge of concepts, shortcuts, and strategies?
Did you take at least four or five practice tests with a progressively higher score [though I know there are swings between different test prep companies]?
Did you regularly visit this site and share your problems with others in order to see other ways of solving them?
Did you also try to solve the problems others posted?
Do you feel you have exhausted the GMAT prep materials available to you (Princeton, Kaplan, Official Guide, Manhattan GMAT, this site, other sites and resources)?
Do you feel that you were hard on yourself even when you were doing well (don't even allow yourself to assume you are doing well just because you solved 20 problems in a row correctly)?
Did you study using a system (when to study, how much to study, what to study, and on what days to study it) or did you study whenever and however?
Did you come back to the problems you could not solve/verbal questions you did not answer correctly (you have to dwel on your mistakes, otherwise you will not improve)?
Were you nervous when you sat for the exam? Did you prepare well (food, stamina, the call of nature [bathroom], sleep, etc)?
Did you shortchange yourself by trying to take the exam sooner, rather than studying longer and more effectively and taking the exam later?
There are many other questions you could ask yourself, but these are some important questions to consider. I am not going to pretend to be a pro, here, since I have had to consider all of these issues at one time or another, and will have to consider other issues in the near future.
Best of luck to you... however, try to minimize the luck factor and maximize the "I kicked ass because I prepared" factor.
