What often happens when we try to cast longer distances is we overpower the cast, "hit it" too hard in an effort to put more energy into it. This then manifests itself as a tailing loop. Without watching you cast it is difficult to know exactly where this is happening during your casting stroke. The solution to all tailing loops with longer casts is to increase the distance the rod tip travels (use a longer casting stroke) and make sure that you are applying the power smoothly with a gradual acceleration to a stop. Also, you need to be very smooth with your double haul, as any jerkiness can also cause a tail.
The best video I've seen that teaches distance casting and helps you clean up tailing loops while distance casting is George Roberts's Saltwater Fly Casting
. I have it and can recommend it highly. You can reach George through his website:
Another good one that I've grabbed recently is The Art of Advanced Fly Casting
with Chico Fernandez.