Profit is almost always proportional to risk, and in some investment cases to effort as well. I know many writers who invest their AdSense earnings in domains and hosting fees so that they can get more of their own sites going, where you get to keep the whole pie instead of just a percentage of earnings, and that hubs can help you promote. I personally use about half of my AdSense for domains and their upkeep, but have just started doing it recently so the payoff is relatively small so far. If you're looking more for longer term, invest-and-forget options, the lowest risk is always in passbook savings accounts, then goes up to CDs and money market accounts (none of these options are paying much at all anymore, but they are VERY secure), on up to the stock market and finally to peer-to-peer lending. The last is BY FAR the most risky, but if you know what you're doing it can pay off very well...I'm currently getting an average of 22% there, but there is an extremely high risk of loss and little recourse to recover it.
As far as how to get more in writing...pb3847 is right on that, though thousands of words that aren't properly targeted can end up being a waste of time. I personally aim for about 3,000 words I get to keep and 3,000-4,000 words for clients every day. I don't have anyone else writing for me, but that's what the clients pay for.