Not just numbers
Just to be anal dogs actually have about 50 time the 'smell power' of us. Humans have about 5 million scent receptor cells where dogs have about 200 million.
Dogs can actually detect odours at 10,000 time less concentration than humans can; not only do they have more receptores, but their threshold for detection is much lower than humans. Those combined give them much more ability than humans.
As for actually evading detection, I've found one method that works (for food - don't have access to a sniffer dog!). My friend has a dog that can detect food in your pocket, long after you've forgotten about it. So to test this out, I first of all soaked a cloth in clove oil and let the dog sniff this first. Result - dog's sense of smell was almost disabled; the volatile elements of clove oil act as a local anaesthetic (that's why clove oil relieves toothache) which obviously prevent transmission of the nerves associated with the receptors in a dogs nose. What's the bet that they start training dogs to detect clove oil!
Dogs that can detect MDMA, don't actually smell the MDMA, they smell the starting ingredients that haven't been removed (no quality control for illicit drugs!). If you ever get the chance to smell either safrole or 3,4-methylenedioxybenzaldehyde, you'll know what I mean, as they're used in the perfume industry.
PS. The above starting material can actually diffuse through the polymer used for plastic bags as they're soluble in non-polar solvents. It's just a question of time