Big question you are asking here! What does "successful" even mean? The answer is very personal, and yours will probably even change over time, just like mine already has a few times.
I think the traditional answer has been centered around 3 main dimensions:
1. First obviously, the intial training (school), which largely decides what type of job you are going to do: easier to become a Doctor if you study medicine in school rather than arts or astrophysics
2. Second, you need to do well in your actual job. Assuming "successful" means "getting promoted", you need to do you job well: keep your boss happy by taking care of your assignments, making his/her life easy by anticipating issues and coming with options/solutions before you have to
3. Third, you need to be proactive in finding new personal growth and promotion opportunities
A 4th dimension is often forgotten: you need to be lucky! Luck plays a role in how your bosses and mentors are, whether the company grows or sinks... All/most of us who got into MBB were lucky too btw.
Above and beyond these initial responses, I would also like you to consider the following books:
a. 'Give and Take', by Adam Grant, a star Professor at Wharton. If you aren't familiar w/ him, I suggest you read his research: very accessible, fun to read even, and a valuable take on the need to help others versus only only looking for personal benefit. Quick aside btw: I would argue that 99% of the experts on this site are largely driven by this desire to give back. Yes we get paid when doing a case, but the pay isn't very good when brought back to an hourly rate; helping applicant get into MBB is priceless however!
b. 'Grit', by Angela Duckworth (also UPenn). Another approachable book that everyone should look at. The idea here is that IQ, academics or family wealth/connection aren't the most important factors of success. Instead, people who succeed are the ones who just don't give up. That's it: don't give up, keep trying :)
"Success" is not an overnight factor, life is a marathon. Do you best, keep learning, be open to new growth opportunities....
PS: I would argue being on this website and trying to figure out how to better your prospects is already a pretty strong indication of your character.
Hope this helps.