This site has some good information:
Constipated Dog-Dog Won't Poo
Constipation: A constipated dog spends longer than usual defecating (eliminating, pooping), and the resultant stools are small, round, and hard. You can bet that if your dog is constipated, he's uncomfortable.
One of the main causes of constipation is insufficient water, often coupled with too much time between outdoor potty breaks.
How to help your dog
• Make sure your dog always has plenty of fresh water to flush her intestinal tract.
• Add vegetables to her diet.
• Give plenty of exercise, either walking or playing fetch or anything which will stimulate her bowels.
• Offer canned pumpkin in its pureed form (not pumpkin pie filling)
• Give one teaspoon full of salmon or olive oil for a dog but much less for a puppy.
• Offer a small bowl of milk.
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Using a warm (not hot) moist wash cloth, gently apply slight pressure on her tummy while stroking downward toward the anus. This action mimics the natural way a mother dog uses her tongue to help her pup to eliminate trapped gas while encouraging the elimination of waste and urine. If you will be away from home for a long time, arrange for a neighbor or a professional pet sitter to let your dog out to relieve himself. Never withhold water from your pet as this could set him up for kidney problems and other behavioral issues. Prevent constipation by adding vegetables to his diet. Many people use a combination of cut green beans mixed with wetted dry dog food.
Exercise is a great constipation cure. Depending on the age and health of your dog, a long slow walk for older dogs or a vigorous game of fetch for a younger, healthy dog, does a great job of kick-starting a sluggish bowel and will have your dog as regular as clockwork in no time.
Milk will often help the constipation problem as dogs even reluctant to drink water will most often drink milk if offered. Offer a small bowlful and let him lap it up. He may initially have a runny stool but it should solve the constipation problem.
Canned pumpkin in its pureed form (this is not sweet pumpkin pie filling) is also a great stool softener and a natural remedy for constipation. It helps soothe an upset stomach or indigestion in both cats and dogs. It is very rich in fiber and adding just one or two teaspoonfuls to your pet's food often gets their system moving in no time. Dogs may eat it by itself and that's all right too.
DO NOT give laxatives to a constipated dog! Over-the-counter laxatives are more likely to do harm than good. Laxatives cause diarrhea and diarrhea causes dehydration. Dogs can become dehydrated much more quickly than humans and laxatives may contribute to gastrointestinal discomfort as well.