No matter how careful you are, accidents are bound to happen. That's just the way it is with puppies. If you catch your Lab in the act, it's okay to say “Aaagh! Outside,” but avoid using the N-word (“No”) or saying “Bad dog!” It's the place he chose that's bad, not the act itself. Anger and punishment only increase stress and fear, not to mention a greater likelihood of more accidents.
If you find a puddle or pile after the fact, simply clean it up. It's too late to scold. Never use the long-discredited practices of rubbing the dog's nose in the mess or swatting him with a newspaper. That only teaches a dog that you're a big bully who attacks him for what is, after all, perfectly natural behavior.
Keep a good enzymatic cleanser on hand for puppy clean-up. These products contain enzymes that break down organic debris and waste. Products that do a good job include Resolve carpet cleaner, OdorMute, and Nature's Miracle. Look for them at your grocery store or pet supply store. Before using any such product, test an inconspicuous area of the carpet (or other fabric surface) first to make sure it's colorfast.
Avoid using any cleansers that contain ammonia. It's one of the components of urine, and the scent of it will draw your puppy back to the same spot. Stick to cleansers made specifically for cleaning up pet accidents.
How to Clean Up After Your Dog
When you find a puddle, use an old towel to soak up as much of the wetness as possible. Then saturate the area with the cleanser of your choice. Lay a clean towel on the spot and cover it with some heavy books or another heavy item. This helps wick the moisture out of the carpet.
For a pile of poop, use a plastic bag or a towel to pick up as much of the mess as possible. Spray the area with the cleanser and use a clean towel to blot up any remaining stool. Then follow the steps described above. When the area dries, it should be odor-free.
Keep lots of old towels or rags on hand. You're going to need them. Dump used towels in a pail in the laundry room and wash them in hot water so they'll be ready for the next accident.