You really want to take the riba with fat - it needs the fat to bind to or you will just lose it out of your system. Riba must build up and get to a certain set level in your body - absorption is crucial. I personally would make sure that I was taking it twice a day with something that was loaded with fat no matter what you eat for the rest of the day.
You also don't want to take it with Tums or Maalox or something like that because it will have the same negating effect.