In the lucrative world of TV DVD, there’s never too much of a good thing.

And now that some of the biggest-selling series, such as Sex and the City and Friends, have released all of their seasons on DVD individually or in box sets, home video marketers are hardly wringing their hands in despair.

Instead, they’re keeping all these series in the spotlight by repackaging and repurposing episodes in themed single-disc compilations.

Three new Friends DVDs are out (Warner, $15 each) that allow fans to relive favorite themes on the smash series: The One with All the Birthdays, The One with All the Weddings and The One with All the Babies. Each disc has four episodes in which cast members, well, celebrate birthdays, get married or encounter babies.

Similarly, now that all six seasons of Sex and the City are out on DVD - and a $300 collector’s set with the entire series was rushed out right before the holidays - HBO Video has created a line called Sex and the City Essentials, compiling the themes The Best of Lust, The Best of Mr. Big, The Best of Romance and The Best of Breakups. Each disc contains three episodes and costs $10.

"It’s classic catalog marketing," says 20th Century Fox Home Entertainment’s Steve Feldstein. "You never run out of product."

Six years ago, Fox was the first studio to roll out "complete season" sets of a popular TV show, The X-Files, which ran from 1993 to 2002. After all nine seasons were released in pricey box sets, Fox last summer launched a new line, The X-Files Mythology, which repackaged episodes along story arc lines.

So far, four volumes have been released: Abduction, Black Oil, Colonization and, most recently, Super Soldiers. Each volume retails for $40.

Feldstein says Fox sold more than 2 million combined units of the first nine complete-season sets of The X-Files and "several hundred thousand additional units" of the four themed sets.

"In creating The X-Files Mythology, we’ve discovered a whole new audience," Feldstein says.

Fox applied a similar strategy to cult TV series Buffy the Vampire Slayer. The seventh and final season hit DVD in November 2004. The following March, Fox went back to the well for Buffy the Vampire Slayer - TV Starter Set, a single disc featuring the series’s first two episodes. That was followed last October by Spike - Love Is Hell, with four episodes that focus on Spike (James Marsters).

Later this year, M*A*S*H will end its 11-season rollout, and Feldstein says the same strategy may be applied to that classic ’70s series.