Buy high and hold forever? Maybe that's the best course of action when it comes to fast-growing, high-quality companies. Not every stock that hits new highs will continue to generate market-beating returns, but best-in-breed companies could continue their winning ways. Our investors think Weight Watchers (NYSE:WTW), Shopify (NYSE:SHOP), and Align Technology (NASDAQ:ALGN) could still see their share prices climb, despite recently doubling.

It's not the stock that's slimming down

Rich Duprey (Weight Watchers): Weight-loss specialist Weight Watchers hit gold when Oprah Winfrey signed on as an investor two years ago. With a diehard built-in fan base, and her regular documentation on social media of her progress on the Weight Watchers program, the company had built-in free publicity. Its shares took off.

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Weight Watchers shares have doubled in value since last August and have quintupled over the past year. The amazing thing is, there's no reason to think it won't continue to appreciate.

Just this month it signed music impresario DJ Khaled as its social media ambassador. With a massive following on Twitter, Instagram, Facebook, and more, DJ Khaled will be following the Oprah model of recording his progress on Weight Watchers' newest meal program. What makes it more interesting is that he can bring in an entirely new demographic for the company.

Weight Watchers was already seeing tremendous success with the way its diet plans have evolved. From a strict regimen to one that assigns points to various foods, Weight Watchers has seen its membership rolls grow. When it unveiled its new Freestyle program in December, it was instantly declared a hit because it made certain foods "point-free," meaning dieters would no longer need to count them.

That sort of plan would naturally be popular with its base, which has tended to skew toward older, higher-income Caucasians, but now that DJ Khaled will be following Weight Watcher's Freestyle program, he should be able to bring in younger, more urban customers.

From around $10 a share a year ago to over $60 today, there's good reason to believe the weight-loss leader's stock will continue its torrid pace.

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Shopping for profits

Danny Vena (Shopify): In a world that is increasingly dominated by e-commerce, business owners have better things to do than set up and update websites, process payments, negotiate shipping terms, and track orders. That's where Shopify comes in, providing a one-stop shop to handle all these things and more.

The company created a platform designed to simplify the task of setting up and running an online store by providing the technology, tools, and resources that most small and medium-sized businesses lack. Shopify has hundreds of templates to choose from for creating a website, and a variety of other options to streamline the process.

Merchants can customize their website, integrate with all major payment processors, and link to the largest third-party logistics and shipping companies. Shopify also provides cash advances to qualified merchants based on their sales volume, sidestepping the typical bank approval process.

In its most recent quarter, Shopify generated revenue of $171 million, up 72% year over year. The company lowered its operating loss from 9.5% of revenue to 7.4% of revenue, and produced an adjusted operating profit of $1.7 million -- the first time since going public in early 2015.

The company's growth has been staggering, boasting 500,000 merchants in 175 countries, and exceeding 70% year-over-year revenue growth in every quarter since going public. Shopify's stock price hitched a ride on the company's financial results, more than doubling in 2017 and gaining 340% since its public debut in the second quarter of 2015.

I think the best is yet to come for Shopify -- and its investors.

The reasons to buy are clear

Todd Campbell (Align Technology): Orthodontists still use metal braces on most of their patients, but Align Technology's Invisalign is becoming more common now that improvements support its use in teens and complex cases.

Align Technology's Invisalign is the leading transparent teeth-straightening product, and rapidly growing sales worldwide have caused its shares to quadruple since 2014. Despite growing demand, however, the company has still only secured less than 10% market share, suggesting there's plenty more room for growth.

ALGN Chart

ALGN data by YCharts.

Management hasn't reported its full-year 2017 results yet, but the company's third-quarter performance shows that momentum isn't slowing. It shipped more than 236,000 Invisalign cases in the quarter, up from 178,000 the year before. It had sales of $385 million and profit of $83 million, up 38% and 61% year over year, respectively.

That's envy-inspiring growth, but the potential to sell more cases overseas and penetrate the teen market more deeply has me thinking that Align is only getting started. International sales skyrocketed 47% year over year in the third quarter, nearly doubling growth in North America. As key global markets, including China, get increasingly wealthy, I expect international sales will continue setting the pace. It's also important to realize that while Invisalign is getting more frequently used in teens, its market share in the demographic is a paltry 4%. As more practices buy Align Technology's scanners and get more comfortable using it in younger patients, I expect Invisalign will be used much more frequently than it is now.

Admittedly, Align Technology's ascent isn't likely to be a straight line. It was among the S&P 500's best performers in 2017, and there's bound to be stumbles in its stock price from time to time. Nevertheless, enviable brand recognition, robust bottom-line growth, and a massively underpenetrated market position suggest its shares will be higher in the long term.

Danny Vena owns shares of FB and Shopify. Rich Duprey has no position in any of the stocks mentioned. Todd Campbell owns shares of Align Technology, FB, Shopify, and TWTR. His clients may have positions in the companies mentioned. The Motley Fool owns shares of and recommends Align Technology, FB, Shopify, and TWTR. The Motley Fool has a disclosure policy.