Our DD also has the same problem. I second the suggestion of lots of fluids, although our doctor told us that milk can be very constipating, so try extra juice (pear juice is especially good for us; we find it in with the baby foods) and water throughout the day (DD will drink more water if there is an ice cube or two in her cup). If you increase the amount of fibre in your diet, you need to also increase the amount of fluids to help move things along.
I personally limit the juice to breakfast time only due to the sugar content, max 4 or 5 oz (usually diluted, unless she is really constipated). We have just recently started giving undiluted juice (DD is 24mo). I also find that raisins, although they are fruit, are very constipating for our DD due to their high iron content, I guess. I was putting raisins on her whole wheat & fibre cereal in the morning and wondering why she was getting more and more constipated. When I cut out the raisins, things worked quite well.
We still buy the baby prunes (pureed) found in the baby food aisle of our supermarket. Our DD loves them, and they work really well. Not sure what I'll use when she doesn't want to eat "baby" food anymore!!
Good luck!
Carla