There are no guarantees per se on getting a permit that I am aware of.

I generally send in my fax on November 1 for a March trip and hope for the best, though I have travelled to the office and landed one in person a few times (same day even).

Odds improve if you can take a middle of the week trip and, theoretically, if you go in person the first day of the fifth month ahead. Non-corridor trips don't have the pressure, too.