Our regular grooming kit
(clockwise from far left)
For tougher knots, mats, and "cotton coat"
For the occasional heavier knots, mats, and "cotton coat", we also use a wonderful tool called a Matbreaker, manufactured by APC2000, Inc and available online from Cat Claws. The Matbreaker looks like a comb with 6 curved stainless steel teeth with sharpened edges. The teeth can be removed and reversed for left-handed use. We recommend buying two and setting one for left, one for right, so you can work on any mat! Replacement teeth are also available.
Look in the pet store grooming aisle for a comb with rotating teeth. These can work well to keep the knots away (though the don't work well on a stubborn "mat")
You may also have good luck woith something called an "undercoat rake". This tool has half-inch widely spaced plastic teeth and can work wonders to remove "low-lying fluff" before it knots!