Back in the last century, ensuring that customers knew you existed involved buying a billboard alongside a busy highway, or leasing space for your coffee shop next to your town’s biggest office building. It was simple.

But in the 21st century business ecosystem, there are three powerful players that can either help new customers connect and find you, or ensure that you’ll lose market share to your competitors. They are Google, Yahoo, and Bing, the most-frequently-used search engines in the United States. (About 65 percent of Internet users start their searches at Google.com, according to the measurement firm comScore.)

Showing up prominently in their search results has become an important part of basic sales and marketing strategy for businesses – but one that isn’t well understood.

“Lots of companies invest in building a great website, but they don’t pay attention to the factors that drive you up in a search engine’s rankings,’’ says Jeff Demers, director of search marketing at Wakefly Inc., a consulting firm in Westborough. Demers says that about 50 percent of Google users will click on the first site that shows up in the search results.

There’s a whole community of consultants in the Boston area that peddle advice about how a business can improve its position on various search engines. The field is typically called “search engine optimization,’’ or SEO. (“Search engine marketing’’ is what businesses do when they buy ads on sites like Google, to be guaranteed that they will show up when a user types a particular search term. SEO doesn’t involve paying for ads.)

Last week, I asked a handful of local SEO experts for their best advice on how a business can raise its profile on sites like Google – and whether it’s necessary to hire a consulting firm to help you. Here’s what they said:

Valid links. Getting other sites to link to yours is a surefire way to raise your ranking on Google and other search engines. But you can’t buy links, explains Dharmesh Shah, cofounder of the Cambridge marketing software firm HubSpot Inc. “Hiring a firm to get a massive number of links is a bad idea,’’ he writes via e-mail. “What many of these firms do . . . is leave comment spam on blogs (with a back link), submit to worthless directories, put links on their own low-quality websites, and any number of other tactics.’’ When Google sees that lots of sites with low credibility are linking to you all of a sudden, it tends to lower your ranking rather than raising it. Far better are links from well-read blogs and media sites.

Fresh and useful content. So how do you get those sites linking to you? “If you create content that is helpful, people will link to it,’’ says Akshay Vazirani, the founder of Boston-based Dreaming Code Inc. If you’re a veterinarian, it may be occasional reviews of new kinds of pet food and treats; a realtor might start a blog collecting advice for people preparing to put their house on the market. “You do have to think more like a publisher or a media company, as opposed to just building your site and then letting it sit there”.