by Steve Ryfle It's tough being mega-famous. One minute you're on top of the world, the next you're yesterday's it-girl, counting your millions, waiting for the phone to ring, and asking yourself: What went wrong?

For most stars, celebrity has a limited lifespan, and in a business that replaces today's hot ticket with tomorrow's in a heartbeat, one false career move or a few tabloid shenanigans can shorten that lifespan. Suddenly, an upward career trajectory is looking decidedly downward.

Two cases in point: Britney and J-Lo.

It seems like yesterday when Britney Spears ruled the charts with "Baby One More Time" and was proudly showing off her brand-new bustline on Nickelodeon's Kid's Choice Awards. Those were more innocent times, long before Britney shared saliva with Madonna on national TV, "acted" in that unwatchable "Crossroads" movie, married-then-unmarried her sad-sack childhood sweetheart, and then announced her engagement to... a backup dancer.

And wasn't it just recently that we thought Jennifer Lopez was a decent actress, based on "Selena" and especially "Out of Sight"? Even her "On the 6" album was inoffensive, if not exactly good. And "Angel Eyes" and "The Cell" were okay, but... then came Puff Daddy (and the black cloud of a shooting incident), a song that had the n-word in it, a string of flops: "Maid in Manhattan," "Gigli," and "Jersey Girl" - and, of course, the pink diamond and cancelled wedding insanity of "Bennifer."

They've never really gone away, and their careers are still relatively young, but Spears and Lopez already seem ready for the comeback trail. No one knows if they can bounce back to the top of the A-list, but it seems certain they'll try.

"If you're a celebrity, it's your job to be famous; most people try to hang onto their jobs--especially ones that pay really, really well," says Joshua Gamson, author of "Claims to Fame: Celebrity in Contemporary America" (University of California Press).

"Most of the people who become famous are the sorts of people who like a lot of attention--it's a self-selected bunch, from the get-go--and there's no reason to expect they'll suddenly become different personality types," Gamson says. "Most importantly, celebrity (the capacity to attract attention) is a very valuable commodity in this culture. You can trade it for money, for one thing."

Spears and Lopez could learn a few things from a celeb chameleon like Madonna, who has sustained a 20-year run as a superstar through deft self-reinvention and pop sleight of hand. Neither the scandals over her "Sex" book and her "Like a Prayer" video, nor her widely panned performances in films like "Dick Tracy" and "Swept Away" (for which she received a Razzie) could sink her very successful ship.

Of course, few stars enjoy eternal stardom, no matter how popular they once were. Just ask a few actresses in the vicinity of 30 and beyond. Conventional wisdom might say that Cameron Diaz, Sandra Bullock and even Julia Roberts had their turn at the top and are now showing signs of fading - as Geena Davis and Melanie Griffith have, and as Sally Field and Jill Clayburgh did before them. Meanwhile, worthy (and younger) replacements (Kate Hudson, Reese Witherspoon, Brittany Murphy) are following in their footsteps, thank you very much.

A superficial, non-scientific survey seems to show the fickle finger of fame is somewhat sexist, as the guys have more longevity. Some actors who started out in the 1960s and 70s are still going relatively strong: Nicholson, Pacino, DeNiro, Hoffman. Most of the actresses who co-starred with them back then, such as Cybill Shepherd, Karen Black, and Katharine Ross, aren't.

Nora Ephron once quipped, "There are five stages of fame: denial, anger, negotiation, acceptance, and death. These stages are virtually the same as the five stages of terminal illness." She could have added a sixth stage, the turning point when a red-hot star begins to flame out. Kind of like when a TV show "jumps the shark." The problem, though, is that it's nearly impossible to known when someone has "jumped the shark" until it's too late. Oops--career over.

As the venerable Burt Reynolds once put it: "First it's 'Who's Burt Reynolds?' Then it's 'Get me Burt Reynolds.' Then it's 'Get me a Burt Reynolds type.' Then it's 'Get me a young Burt Reynolds.' Then it's 'Who's Burt Reynolds?'"