1) Try NOT to hit any of the walls or barriers even if it means slowing down at the corners
2) improve your gear shifting sequence. ie: try not to "drag" any particular gear for too long if you dont notice any speed increase
3) practice
Other than that, there really isnt any secret. Just more practice by going fast without hitting the walls and you will notice your pick up speed will be faster in the straights.
If using a DC2, you should be hitting at speeds of 180kph before the finish line if you doing it without hitting walls at the last bit.
Check out kohanson's strategy and speed pic. It is very useful.
Hope this helps.
