A celebrity product endorsement is like a marriage. Some matches just make sense: We all believed that Bill Cosby was nuts about Jell-O pudding, and clearly the pudding loved him back. But too often, the pairing feels very, very wrong. Was anyone happy to see Bob Dylan's mug in an ad for Victoria's Secret?

Clearly, companies need help. Enter matchmakers like Jarrod Moses. The 42-year-old founder of Manhattan marketing agency United Entertainment Group paired Covergirl with Ellen DeGeneres and Build-A-Bear with Victoria Justice. He's one of those annoying overachievers who puts in 12-hour days at the office before heading home to spend another four hours jabbering on the phone. There's a lot more to the matchmaking than folks imagine, he says.

According to research firm Millward Brown, the share of ads featuring celebrities fell from its peak of 18 percent in 2004 to just 11 percent last year. But when brands and celebrities do hook up these days, Moses says, the relationship runs deep. A "360-degree package" might include blog posts, tweets and attendance at sales calls. In return, the celeb can demand stock options or a percentage of sales. A multiyear deal can pay tens of millions.

Companies can have quirky demands. A CEO might request a partnership with a favorite star just so he can meet his idol -- never mind whether it's a good fit. Celebrities are even more difficult to handle. One minute they're national heroes; the next they're mired in scandal, dragging the brand down with them. "You have no idea what will happen," says Moses.

Moses recently worked with Frito-Lay to find an entertainer for its new chip flavor, Doritos Late Night All Nighter Cheeseburger (don't ask). It didn't take long to generate a list of possibilities. The universe of commercial-ready stars is smaller than you'd think -- for any campaign, you're looking at maybe 20 candidates. Moses and his team further narrowed the pool based on experience and gut feeling: They know many stars personally and have a sense of who's a good fit.

There's some science to it as well. Moses uses a data service that rates stars on factors like name recognition and whether folks perceive them as "boring," "rude" or "a good listener." Moses thought pop star Rihanna looked like a match for Doritos. Her profile is high among the Doritos target audience of 18- to 24-year-olds. Plus, she scored well on attributes ("trendsetter," "confident") that Frito-Lay wants to associate with the brand. As luck would have it, Rihanna was intrigued: She liked the idea of a music video that consumers could access by scanning a bag of Doritos with their webcam.

The deal looked great on paper, but would it gel in real life? Moses always arranges for a celebrity to meet with company execs to make sure the chemistry's right. Half the time, he says, the deal falls apart after the first date. In this case, Rihanna and her handlers met the Doritos team in Miami just before the Super Bowl. Then came the fun part: four months of negotiations with agents, publishers, the record label and musicians.

In this case, the pairing paid off. Within months, bloggers were quoting Rihanna gushing about her venture, the song was a hit, and the cheeseburger-flavored chips were flying off store shelves. Frito-Lay says the collaboration "helped raise the profile of the flavor." It's not a cure for cancer, but at least no one's embarrassed. Now could someone ask Charles Barkley to stop shilling for Weight Watchers?