Training Facility (most important)
I did a lot of bouncing around and training at schools that were subpar because they were inexpensive. I made a huge 2 year mistake at one school where I continued to train because I felt guilty and it was essentially free. I wish I had valued my time more than money but what can you do when you're young - BJJ is expensive. I tried to make the most of it but at a certain point you are no longer the limiting reagent.
I also wasted a ton of time trying to find people to drill with. If I was at a school where people were already self motivated to drill this would have been easier.
Mentality (second most important)
"Relax" is usually good advice for most people but it was not for me. All the way up until about a little past blue belt I was too relaxed - I was always trying to flow roll. Hindsight 20/20 thing but I think I would have gotten more out of my rolls if I had maintained a base level of intensity. I believe Bernardo Faria suggested that one should roll as hard as they can without getting gassed or hurting themselves or their partner; I believe this is the ideal way to roll. Then again, maybe I needed the loosey goosey years to develop better feel and technique. Maybe the more intense rolls are only useful once you have some tech under your belt.
Being too relaxed also got me hurt a lot. I now always keep the boxing mantra of "protect yourself at all times" in my head. Sometimes I forget it, get injured, and need to be reminded of this lesson.
At this point in time I have trouble comparing myself to black belts. DirtyHolt seemed to suggest to me that when he began to see himself as a challenger and perhaps even a equal to other (high level) black belts, his game turned a corner. I'm going to try adopting this mentality.
Off mat work (e.g. lifting, flexibility, mobility, etc.)
I need to lift. It is not an option for me. I treat it like brushing my teeth - I shouldn't be even talking to people if I haven't brushed my teeth. I was always 150lbs, which I think is the minimum weight one can be to handle bigger people only because I've seen Rafa do it. I will never forget his nogi match with Roldofo Vieira at ADCC. Nothing really happened but it blew my mind that Rafa was able to hold him Vieira off and even give some back. Rafa is a physical specimen though. I thought to myself "if Rafa can fight monsters, maybe I can too, if I lift". Let's ignore the other factors that make Rafa a beast for now.
I don't even see lifting as an offensive measure, as something that will make my takedowns better or my chokes tighter or whatever. It is purely a defensive strategy. By that I mean that it cushions my joints and allows me to better support my frame. I went down to 135lbs after a surgical procedure and no offense to people that weight or smaller but I felt extremely vulnerable despite only a 15lb weight drop. I felt like my skeleton couldn't even support other peoples weight when framing.
Furthermore, lifting is just good daily maintenance (like brushing your teeth) and allows you to counterbalance all the pulling and awkward positions bjj has you do.
BJJ also took away a lot of my flexibility. I realized I needed to continue to work on my mobility and flexibility to be safer, perform better, and just be healthier overall.
I would mention sleep and diet but I don't find these things to be as important for me.
TLDR:
1. Go to a good (i.e. competitive) school
2. Enter each roll with the proper mentality
3. Don't neglect off the mat work like lifting, stretching, and mobility.
Also obviously these 3 things are unique to me. I also think they if #1 is true, then just about everything else will happen by osmosis.