Obvious idea is obvious: Save and invest a very large percentage of your income - I'm at 25%, but I'm not very ambitious. At 75% you can retire for three years for every year you work, even without assuming any gains from investment income or any other sources of income. If you are 30 and reasonably established in your career, this means you can work for ten years and then retire.
Combine this with the idea that the most worthwhile investment by far is investing in yourself. Take whatever percentage of your income and use that to help you learn skills that will increase your income. (This includes not just technical skills, but self-marketing skills, self-confidence, anything. You will probably have more and more ideas the further you progress.) Most likely, you will have a much greater return than if you simply invested the same money in more traditional investments like the stock market.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.