There is a drug on the market named ELMIRON which is pentosan polysulfate sodium and used to treat this condition. It is made by ALZA Pharmaceuticals formerly Baker Cummins. You can look this up in the CPS at your drug store or better yet, speak to your physician about this treatment.
Check the Internet using AltaVista and look up Baker Cummins. You will find a great deal of info. on Interstitial Cystitis and Elmiron.
I find that diet doesn't eliminate, but greatly reduces the pain. No acids (that includes alot), no alcohol, no vinegar based salad dressings.