Go Into Sales
Let's say you're a college student who wants to be an entrepreneur. Let's also say -- for one reason or another -- you don't want to jump in and dedicate all your time to an idea right now. Maybe it's money, maybe it's confidence, maybe it's the lack of available co-founders or some combination of the above. From what I've seen, most people in this situation do one of the following: 1. Join a big company and plan to do your startup later.
2. Join another startup as a non-founder.
3. Go to graduate school.
I have a different suggestion: go into sales. Take a commission-heavy sales job at a company that gives you the ability to source and manage your own leads with as much independence as possible. Find a larger company in your space of interest and just go learn to sell things there.
The ability to sell is the most underappreciated startup skill. In the get-bought-by-Google model, you just have to be able to code and (possibly) market a product. Ideally, you build something so awesome it just takes off by itself. Sadly, this rarely happens. Instead, almost all companies have to sell something at some point in their lives. And this isn't a bad thing. It can keep your company (and you) independent of venture capitalists and other control-seeking investors. Having a fundamental understanding of the psychology of sales and the sales process can be the difference for your startup.
Also, sales isn't IBD analyst-style slave labor. There's literally no cap on the amount you can make, which means that you can potentially bring home a decent six figures a year if you're really good -- way more than most first- or second-year analysts in investment banks. The hours can also be ridiculously flexible; if you'd like, you can work hacker hours.
For whatever reason, sales jobs are off the radar of most Ivy League college graduates. Since a nice degree doesn't mean anything in sales, little to no recruitment is done on campus. And Ivy League undergrads love to be recruited.
There's no better training for being an entrepreneur than actually starting your own company. But if -- for whatever reason -- that's not feasible, sales is an good and woefully underappreciated route.