These are questions I've been asked only once or twice, but which were thought-provoking enough that I want to post them anyways. This idea was taken from Paul Graham.
1. How did/do you have so much time?
Mostly by cutting out activities that aren't important to me.
I feel high school students tend to focus too much on "local" changes (using Chrome productivity extensions, shaving time off X, etc.) than on "global" changes (dropping entire classes or commitments). It's not that local changes are bad, but you should usually consider global changes first before local changes (80-20 rule). Aggressively dropping classes and extracurriculars that aren't adding value to your life are good examples that I think people don't do enough.
(I admittedly have spent a lot of time optimizing my workflow, like typing quickly, using Vim/Linux, organizing tasks on calendars, but I think these are second-order effects.)
I took this to extreme measures in high school. As a senior I took a total of 2 regular high-school classes and spent on the order of 30 minutes on homework per week; as a result I had enough time to do math, write a geometry textbook, take graduate math classes, learn to play this piece, sleep a ton, AND play StarCraft. While I don't encourage everyone to do this, I think it illustrates my point.
In general, I think most high schoolers don't spend enough time thinking about exactly what's important to them; this is understandable, since it's an error of omission, and so you make such errors by default. It's especially bad in high school because high school gives you a very clear path of "this is what you should do" by having you go to class every weekday until 3PM, then assigning you the homework to do at home, and giving you letters of the alphabet which amount to obedience scores. But school is rarely optimized for the top students (even at magnet schools), which is why I had the guts to tear up my entire default schedule and replace it with the things I mentioned before.
For those of you who were at Math Prize 2015, Victoria Xia made a somewhat similar point at this year's MPFG Alumni address, which you can watch here, 49:30 -- 55:30.
2. I'm a high-school senior who is not eligible for MOP / IMO anymore. What should I work on now?
I guess the best advice I can give is to pick some project to work on. It doesn't have to be (and probably shouldn't be) something that you're confident that you can do well in. For more on this I much like the advice suggested by Paul Graham particularly in Section "Now".
Maybe it's better if I gave examples of projects that I worked on (both successful and unsuccessful):
- Programming projects (Github)
- Installing Arch Linux and figuring out how to use it
- Writing nonfiction e.g. Napkin and EGMO (the latter was written while I was a clerk at my high school's office)
- Writing fiction stories (which I failed miserably at; tried at least two or three times)
- K-pop dance (not good at it yet but trying!)
- Creating a problem database (this took me five or six tries before it worked out)
- Creating a constructed language (failed at least five times)
Anyways, the point is to pick something that feels cool, or that you've always wanted to do, and then try to do it. I think this often turns out to be easier than you might expect, because you already have a lot of experience learning something that felt too hard for you (through math contests). As far difficulty goes, the optimum is to pick a project with a 50% chance of success. But really the main thing is to optimize for fun, because otherwise you won't be able to force yourself to work on it anyways.
It's understandably very daunting to move into something in which you have little experience. But you don't have to hard-commit to exploration projects as a full-time job, the same way you did for math contests. In other words, don't feel locked in to any one particular thing, and don't pressured to become the top N in the world: after all, the point is really for your own learning/growth/enjoyment.
3. What's the most important thing that math contests taught you?
I have a whole post on lessons I learned from math contests, but here's one that mattered to me a lot back in high school.
In my opinion, one of the most damaging messages I got during high school is that hard work pays off. This is not true, and one of the most important life skills that math contests taught me were how to work hard even being fully aware that I might never "succeed". Most notably, this requires enjoying the work itself rather than just as a means to an end.
In school there's a mentality that teachers should reward "good effort", i.e. if a student tried their best, they should get a grade of A. By extension: "if you work hard in life, you will succeed". This turns out to be quite far from the truth. Math contests teach you this lesson much better: you can love the subject, work harder than everyone else, do everything right, and still not win the USAMO; that's what makes it worth doing.
To quote Paul Graham:
Hard means worry: if you're not worrying that something you're making will come out badly, or that you won't be able to understand something you're studying, then it isn't hard enough. There has to be suspense.
You don't see faces much happier than people winning gold medals. And you know why they're so happy? Relief.
Anyone can work hard if reward is certain, but in real life this is simply not the case. This is why it's so important to enjoy the process itself rather than fixate only on the end result.
With all that being said, I am obliged to add on that the other huge thing that contests did for me was provide me access to one of the best peer groups out there. Most of the friends that I have today came in some way or other from the math contest circuit, even though many of them will certainly not be doing math as a career. (This doesn't technically relate to the question because it's not something I was "taught", but it would have been misleading of me to omit it; the social aspect of contests was huge for me.)
4. Would it help to have a tutor/mentor for olympiad math?
Short answer: yes, but not in the ways people often expect. Long answer: this blog post.