Prunes. Get a packet from the supermarket (I like Angas Park best) and stew them. Put them in a bowl in the microwave with enough water to cover, cook on high for five minutes and leave to cool. After a few hours of absorbing water they'll be soft, and good to add to cereal or yoghurt.
I do a batch a week and keep them in the fridge, hubby has them every morning. I find about four a day keep the stool soft.

My Mum recommended these to me when I had the same problem after my first child, and I've used them on and off since over the years as the need arises - better than laxitives!

Kerrie