I know of others on the site that have not had results on the first flush...or for that matter on some other flushes. I have asked the same question you did about what could be done and I got various answers. Here are the ones that I remember:
1) Drink apple juice, as much as you can stomach, during the week before the flush.
2) Ensure that you are following the flush to a "T". No fat!! I also think that a good diet in the few days before the flush would be a good idea (A good diet is always a good idea).
3) Drink plenty of water every day, especially on the days leading up to the flush.
That is the main advice that I have recieved. The only other thing I can say is don't give up. One unsuccessful flush does not mean that you do not have stones or that youdo not need to do the flush. It would, in my opinion, take several successive flushes with no or little results to prove that. Good luck and post your future results.