Personality and habit. For me, being a graduate student for 7 years (masters then PhD) forced me to live simply. Those habits stuck with me through the next decade of good salaries. My spending went up from graduate student poverty, but I still saved and invested 50% to 70% of gross income. Now I probably have more than a lot of peers who started working well before me, but didn't save as much.

To your point, pay yourself first. Set up automatic investments. Find a fiduciary advisor if you must, but it's really not rocket science.