I'm so sorry it has taken me so long to respond to your submission. I have been on an extended leave due to a death in my family.
Two of my cats are long haired and I have to deal with the same problems. Combing is a must and a regular basis, but I'm sure you know that. There have been times when I actually had to just cut the knots out of their fur, it's much easier to just cut them out, since it can be really painful to try to brush them out. One thing I have used is Olive Oil. In fact I have started to put a light coat of Olive Oil on their fur after a good brushing. It's actually good for them if they lick it off (and they will try to lick it off). It makes their coat shiny and much easier to get a comb through. At first they look a bit greasy but it soaks in fast and is really good for their skin to.