Ok, my hair isn't normally tangle-free to start with, but I've been going through a stage the last month where I can get some ferocious knots in the back. Some have been so bad that I've just had to snip them off.
But like medusahair said, just try and use a good moisturizing conditioner, that should help at least some. Also, I think some products, especially some gels can cause more tangling for me, so I've started to try and avoid putting product in that area, in hopes of avoiding more tangles. The area that is worst for me is the very bottom layer underneath, between the neck and downwards.