To lengthen a drift, tap handbrake and let go of all other keys, once released continue to drift, and tap when you start to lose your drift.
Make sure you are only tapping the handbrake or you will stop.

To end/shorten a drift, attempt to make your angle smaller, and then let go of "W" and attempt to steer out of the drift. The more you release W the shorter your drift will become.

To drift more inwards, tap "S" or your brake key whilst performing a drift. You don't need to worry about letting go of any other keys besides the handbrake.
Don't hold "S" and only tap it when you need to drift more inwards. If you need to continuously go more inwards, spam the "S" key.
Make sure you have enough speed, and to be able to maintain speed before performing this maneuver.

To remain a consistent angle in a drift, perform a Scandinavian Flick[en.wikipedia.org] (also known as an inertia drift from Initial D) and as soon as you steer into your turn, hold handbrake, letting go off all other keys.
This is better done if your corner if only imaginary, and is done on straights. If you weren't already gaining points before initiating this maneuver, you won't gain any points. This is better when used to initiate completely 90 degree drifts. This will only work if you're inwards turn is ~90 degrees and you are counter steering.
There are other ways to initiate this, but this seems to be the easiest. It is actually possible to just counter steer at around 90 degrees and then hold handbrake.

This thread will be updated regularly and more techniques will be added. Hope you found this helpful, and feel free to add any techniques you have discovered or think should be in this down below.
Check out my original post at the forums http://driftstreetsjapan.com/showthread.php?tid=83&pid=369#pid369