I think you will get lots of different responses. What my Bradley instructor suggested: for about a week ahead, eat something sugary each night - she suggested ice cream. Then the morning of the test, eat a high protein, low carb breakfast like scrambled eggs. Her emphasis was on "tricking/ persuading" the test to return "no GD" results so that the OB would not feel the need to start restricting eating and weight gain. Pretty sure there was no scientific basis, but anecdotally it worked for all of us :-)
What my OB said: don't fast. If the test shows a problem we would have done a 3 hour test, and that would have required fasting.