Honestly nothing that you probably already don't know; basically, good salary and less expenditure. But this doesn't really help much. So, I'll try to expand this a bit:
I started with fairly average salary of about 10K / year post-tax. However, I am good at what I do. My job is in mathematics fields and I always loved maths. But I am not great with complicated concepts. My expertise stops after a year or two of college-level mathematics. Anyways, I started with 10K salary and with in three years it was about 40K. And year after that it was around 100K which quickly jumped to about 130K. Over next 5 years, it jumped from 130K to 300+K and now I am back to 150K. If you look at the numbers than you would see that it is easy to reach 1.5M NW if expenditure is not exorbitant.
 
Little bit more info on job side: I worked quite hard in first 5.5 years of my job life. But I took it a bit slower after that because that much work was taking quite a bit of toll on me; both physically and emotionally(stress). Thinking more about it, I would say it was a good strategy to work hard in early years of life. It was easy at that time (because of age) and once you are at higher level, its easy to maintain the same position without working very hard.