My friend's Golden Retriever gets mats behind his ears and near his "arm pits." She uses a mixture of aloe gel and coconut oil in a spray bottle, works it in with her fingers and allows it sit for awhile after spraying on mats. She uses a metal detangler comb for dogs to work out the mats/knots. She said if the mats are adhered to the skin, that a professional groomer needs to address the problem, but if you watch for the mats and treat them early, this helps to prevent them from reaching "critical mass." My dog is a smooth hair dachshund, so I have never personally had to address this problem, but it may be worth a try on your long hair.