Just transited through DEL a few weeks ago. Some tips, which others in this thread have pointed out:

1.) Fly Business or First. It has fast and separate security and a fast, separate passport control line. Without it, you will be stuck in Gen Pop lines....in India.

2.) Print out your entire itinerary, preferably from the airline. Security won't let you in the door without it and I saw several people turned away. Paper boarding passes are also a must.

3.) If you are connecting onward from DEL, you have to pick up your bags and re-check them, similar to a US airport (or at least I did, when connecting within India.)