Your best bet is to avoid processed foods & opt for whole grains, fruits, & vegetables.
ITA. I've noticed that when I binge on something highly processed, I just don't feel right. Now I'm not saying I eat healthy the way I should, but just little changes make a big difference -- brown rice or bulgur instead of white rice, more veggies with meals, fruit with sandwiches instead of chips, etc. Remember that the recommended serving size of meat per meal is like 4 oz (though we usually have closer to 5 oz, or 6 oz with fish). So try to cut your meat intake down to that size, while filling the space left on your plate with the healthier stuff
Also, be aware that starting out raw veggies may be harsh on your system (and bad for your social life

). So don't over do it. Do a mix of raw and steamed/stir-fried.
Another suggestion is to start eating more yogurt. Please don't fall for those super processed yogurts, or the ones with "added fiber" (i.e. steer clear of Yoplait, Dannon, etc). They are full of all kinds of junk. Go for either plain yogurt or if that's too harsh for you, vanilla yogurt with natural sweeteners (Stoneybrook, Cascade, Brown Cow). It is VERY good for the digestive system. Add your own sweeteners with fresh fruit, which gives you some natural, real fiber, or a touch of natural jams (I know someone who loved to add a smidge of lemon curd).
And I have doubts on the benefits of the fiber powders, but for just that little touch of something extra, Benefiber is absolutely tasteless and textureless. Though, I actually have a fondness for their packets of Kiwi-Strawberry drink mix. Yummmmm....