“Smash” is decisively not living up to its title.

It didn’t start out that way. The NBC series, which revolves around the making of a Broadway musical about Marilyn Monroe, premiered in February to 11.5 million viewers, a godsend for a network whose prime-time lineup is holding on by a thread or, more specifically, a mindless singing competition (”The Voice”).

NBC quickly renewed “Smash” for a second season, and executives almost certainly began plotting “Law & Order: Broadway.”

Then fans started tuning out. The show now draws about 6 million a week. Cast members must be dreaming about being booked for something more respectable – like the Spider-Man musical.

It can only get worse. Creator Theresa Rebeck won’t return next season, and a changing of the guard this early in a show’s run is rarely good news. Just ask fans of ABC’s “Commander in Chief,” who threw the Geena Davis series out of office after Rod Lurie was replaced by Steven Bochco.

So what went wrong so quickly – and how can networks avoid making this mistake again? A review of nearly all 15 episodes and a look back at other fast-fading series suggest the following tips:

Don’t oversell

“Smash” had so many ads leading up to its premiere, you’d think Katharine McPhee was running for president. Sure, that helped the initial ratings, but with heavy promotion come weighty expectations – ones that few shows can sustain.

Fox made such a big deal about having Steven Spielberg as a producer on “Terra Nova” that audiences expected “Jurassic Park IV.” Instead, they got a gritty “Swiss Family Robinson.” Needless to say, “Terra Nova” is now extinct.

Viewers don’t want to be told what to love.

Face the music

I’m a big fan of the original songs Marc Shaiman and Scott Wittman have penned for the show.

The ballad “Secondhand White Baby Grand,” introduced a few weeks ago, should be recorded by a pop diva, like, yesterday.

But viewers, especially younger ones, seem to prefer familiar tunes, judging by the success of “Glee,” onscreen and on the charts.

Don’t annoy us

The biggest debate among “Smash’s” dwindling fans isn’t about which character is their favorite, it’s about which is most annoying.

The leader of the pack appears to be Ellis “I just heard something” Boyd (Jaime Cepero), a two-faced assistant who makes Eddie Haskell look like Captain America.

My personal vote goes to the musical’s producer, Eileen Boyd, if only because she’s played by Anjelica Huston. An Oscar winner and the daughter of a film legend shouldn’t be downing shots in dive bars, playing “Big Buck Hunter” and shacking up with a scruffy bartender.

I dare die-hard fans to pick one character to root for. Ivy, played by Broadway vet Megan Hilty, had a great back story – chorus girl seeking a big break – but quickly transformed into a green monster more ferocious than the Hulk.

McPhee’s Karen is the closest thing to a sympathetic figure, but the former “American Idol” contestant doesn’t have the chops to create a three-dimensional character.

Don’t try so hard

The theater world may be fraught with massive egos and backstage betrayals, but “Smash” takes it way too far.

The star loses her job because of a bad reaction to some pills? Check. Switching Marilyns as often as most people change their toilet paper? Check. Watching your lead actor abandon the show two days before the curtain rises? Check.

What will be left to provide drama next year, once the show’s fictional musical, “Bombshell,” has its premiere during the season finale Monday?

Unless NBC finds a way to back out of its second-season guarantee, “Smash” will get a second chance next season. Let’s hope its producers use that opportunity to build a better “Bombshell,” not a bigger bomb.