As a child who's hair always tangled terribly when it was washed, I learned as an adult to a) brush my hair til it's knot-free BEFORE I get in the shower (with a brush with prongs, not a bristled brush, otherwise, it just smooths the top layer and leaves the knots underneath), b) don't rub the shampoo in - use massage-fingers on her scalp like when we go to the hairdresser, and then just squeeze it through the lengths, and comb through the conditioner with a 'rake' (ie, long-toothed, wide-pronged comb)before rinsing, r, and c) to pat and squeeze it towel dry, and never, ever, rub, AFTER the shower. It would have solved many tears and tantrums had my parents known this.

Also, don't let her get it wet unless it's going to be washed - buy her a pretty, well-fitting shower cap, to use every time she's not going to wash her hair. Washing mine every day just makes it flyaway and therefore more prone to getting knotty, even now I only wash it every 3rd or 4th day. If it's going to be windy or damp, tie her hair up to reduce the knotting. If it's in a simple pony tail, you can be fairly certain that the hair pulled flat against her scalp isn't going to be too bad, and you can even brush out the tail before you take the band out.

Oh, and when you're brushing it, do it in small sections, and hold the scalp end of the hair tightly in your hand, so that the brush pulls against your grip when it catches in a knot, and not against her scalp.

Of course, there's always the option of having a much shorter haircut, as well - it doesn't take all the knots away, but it makes them much easier to deal with.