Well, I'm still learning on drifts...But basically what i felt when my drifts doesn't last is mainly because of the entry speed. With inadequate speed on entries, your car normally straightens up fast.
Secondly, if you are able to catch the drift, you need to perfect more on the amount of throttle input when drifting through longer corners. When i first started, i had a habit of over using too much throttle. And when i realise i had too much throttle on, i reacted by releasing too much throttle. This caused drastic over steer in my cases. *Be sure to be in the right gears so the powerband of your car can push it through the long corners. I felt that this was an important yet logic point to take note of.*
Lastly, try to find videos showing detailed level of drifts(steering, brakes, throttle input...etc). They are really useful as they can get you an idea of how does it works! And thats how i'm learning now. We've gota great bunch of unselfish lads here helping each other!
If not, try to post videos and maybe we can point out your mistakes/problems.
Cheers!