My book Prevention's Symptom Solver for Dogs and Cats has a few remedies:
1)Give a little bit of Metamucil (small dogs 1/2 tsp 2x's/day until they return to normal stool passing)
2)Exercise (go for two 15 min. walks/day)
3)Add 2-3 tsp of bran flakes to food once a day until constipation ends.
4)Give 1/8 cup of milk/day until constipation ends.

With my RB Angus the milk and a nice brisk walk almost always worked.

Good Luck!