Overheating of the pad/rotor/caliper/fluid etc or maybe a tired braking hand
A few ways to minimise brake fade. Different pads, or pad backing material can reduce heat conduction into the brake fluid, which will help prevent it boiling. New/better fluid, or vacuuming it before filling, will reduce the amount of air/water/crap in there, which lowers the boiling point. New fluid, especially if you apply a vacuum to it to remove as much gas as possible, will have a much higher boiling point than that oil dark brown stuff currently in the system
Not much you can do about cooling the rotors or callipers, but unless they're going blue you've not reached the limit there yet - most fade is down to boiling fluid.
Possibly the best improvement you'll get it better braking technique (I may be teaching you to suck eggs here...)
Rather than braking softly, but for a long time, you are better off (in terms of minimising heat generating) braking hard, but for less time - constant gentle braking will be continuously adding heat into the braking system, and not allowing it to cool. Less frequent, hard, braking will of course add more heat in a shorter time, but only while the brakes are applied. Two affects from this - the most obvious is that the brake system now has time to cool down between applications, the second is that because the heat was added quicker, it will not have had time to migrate completely into the fluid - most heat will still be in the rotor/pads/(and caliper, to a lesser extent). These items are in the airflow and cool quicker than the brake fluid will, as its stuck in a small, well insulated tube...
Of course, there's a compromise to be made between braking hard and less often, and making sure you don't lock a wheel/go too fast to deal with surprises...
