Fibre is a huge part of how your body detoxifies. It literally cleans you out and takes a lot of toxins along with it on its travels through your digestive tract.
Besides the large amount found in fruits and vegetables and in the usual bran (soya bran is a nice wheat bran alternative) you can also get a great daily helping with psylllium husks (health stores). Just a pinch of the powder over your porridge and add some some extra water, leave to soak for about 3 minutes and your sorted. Amount can be increased or decreased to suit how much fibre you think you'll need daily (a teaspoon is the max amount recommended and an extra glass of water is needed if increasing above the recommended pinch) A pinch is a quarter to an eight of a teaspoon. Don't forget to drink extra water to accomodate the extra fibre.