When I made the jump from D2 to D1, I needed 3 or 4 NT appearances (1st round exits).
With one of my current D2 teams on another ID, I was qualified for D1 after an elite 8 run in my first year at that school. The following season I made the NT again (got upset in the 1st round) and went from 30 D1 openings to 67 D1 openings.
It really is mainly about the NTs and building up. If you have 2PIs and then follow that up with 2NTs, then you are most likely going to be qualified for D1. Recent success, while vague, should include "is your team getting better" in regards to post season appearances. At least that is my unofficial view on it.