I have IBS-like symptoms but it stems from my nervousness, and it's mostly my stomach becoming uncomfortable.
From what I've heard, constipation usually is caused by an unhealthy diet, and not enough water. And years of build up isn't going to be fixed very quickly. Lemme get my uncle's book... "Ultimate Gut Cleanser". Water is a big deal, Moderate exercise, too much meat or dairy can hurt good bacteria, Fermented foods boost good bacteria, poor diet over long time can weaken intestine movements, He says abdominal exercises and raw foods can help fix this. His "Best foods to improve digestion": Sprouted fenugreek, Aloe vera, flax seed water (Glass pitcher, 1 part flax seeds to 4-5 parts water, sit 8 hours, strain seeds out), Flax seed meal, chia seeds (this one alone worked for me), fermented foods, sprouts.
So that's his suggestions. From my experience, avoiding large amounts of cruddy food, eating salads, exercise, water, and chia seeds is enough to fix any poo-problems.