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Some dogs experience some urinary incontinance as they age, especially if they are spayed or were spayed at a young age. The urinary sphincter muscle can be tightened using medication that's not very expensive and it is very effective.
You should see your vet (or give him a call if you have a good relationship with him) and see if he recommends this for your pet.
I bet it will help a great deal.
Aside from that, you should withhold food and water a few hours before bedtime and see if it helps. All of the tricks that help a child not to wet the bed would come into play here.
Let me know if you still have questions.