A well-crafted watchlist is critical to smart investing: It can help you find attractive buying opportunities, and it can save you from rushed, emotional decisions by slowing down the process. The Fool now offers MyWatchlist.com, your free customized hub to follow the performance and Fool news and commentary about the companies you're watching.

But what to put on your watchlist? In the latest entry in our ongoing series, Motley Fool analyst Sean Sun shares a pair of companies he's watching, plus one he recently added to his Rising Stars portfolio.

One to watch
He started with a thesis: There's a lot of money to be made in renewable energy, but which type? Solar had the head start and myriad government subsidies to get the technology in place, but it's still miles from what the energy folks call grid parity, the holy grail of making new technologies as cost-efficient as coal. He came to wind power, which he feels has the juice to achieve that parity on its own merits without governments propping it up. From there, he looked to China as the likely home of an investment in this technology, with the country recently overtaking the United States as the biggest investor in new wind farms and related power projects. Now his sights are set on China Ming Yang Wind Power Group (NYSE: MY), the largest private-sector wind company in the country most supportive of the renewable energy most likely to eventually achieve grid parity. It's a pretty compelling thesis.

But Sean's not ready to buy. The company had its IPO in October so it hasn't exactly established credibility and a track record as a public business. Moreover, his top-down thesis doesn't necessarily lead to Ming Yang as the right answer; he might find winners in the transmission of wind energy or along the supply chain. He's digging and watching to see which specific company is best positioned to profit from this undeniable trend, but Ming Yang is the early leader.

Two to watch
If you're a physician in a mom-and-pop doctor's office, you should be concentrating on your patients' health, not grappling to understand the arcane and complex processes for submitting claims for payment. That's the proposition of athenahealth (Nasdaq: ATHN), a provider of Internet-based business services for physician practices. And while most of the players in the field chase the richest contracts, athenahealth is going for volume. The lower-margin approach seems to be working.

Sean added the company to his watchlist two months ago and started to dig in to get a better handle on the evolving health-care landscape before making a purchase. In that time, the share price has run up more than 40%. And while he recognizes the share price at which he added it to his watchlist is an arbitrary anchor, it's something else to include in his equation.

And one he bought
China Yida Holding (Nasdaq: CNYD) also has seen a healthy climb in its share price recently, but Sean was able to go along for much of that ride when he added the company to his Rising Stars portfolio. And he's optimistic that the growth in this unusual business is just beginning.

Imagine if no roads led to Yellowstone or the Grand Canyon had become a dumping ground, and imagine that there wasn't a government agency devoted to the preservation of these natural wonders. A private company that was allowed to purchase the management rights could come in, clean them up, build the amenities to lure tourists, market them, and turn a tidy profit in this "joint venture with nature," as Sean calls it.

Riding the tailwinds provided by a burgeoning middle class with increasing disposable income and enjoying an incredible operating margin of 65% over the past 12 months, China Yida is poised for success. The share price had tumbled significantly since the summer, and Sean couldn't pass up the bargain the market was offering.

And that's exactly why it pays to watch. You can make smarter investing decisions with your own version of My Watchlist, new and free from the Fool. Click below to start following one of the stocks mentioned above: