Fermented foods (kefir, raw sauerkraut, miso, fermented vegetables, yoghurt,...), prebiotics (rice/oat brans, gums, inulin, FOS,... are needed to build the flora. Yucca root and licorice root powders can help with inflammation.
Search on the Truth in Medicine forum about IBS, you will find useful posts.
I suffer from constipation too, it was VERY severe (had to go to the hospital 2 times), and kefir litteraly saved me. I drink soy milk kefir before going to bed.