It’s not easy for TV writers to think up the perfect ending to a hit series, but "Will & Grace" co-creator Max Mutchnick has a good idea: "Karen bites everyone on the neck and they die, and then she folds her arms over her chest, falls out the window, turns into a bat, and flies away."

Mutchnick might be joking about Megan Mullally’s vampy character, but that could be a funny dreamscape or fantasy of a final "Will & Grace" episode. It would fit the comedy’s offbeat sense of humor.

In truth, Mutchnick and writers on other shows know they face big challenges in coming up with goodbyes for long-running series that are exiting this year — not only "Will & Grace," but also "The West Wing," "Alias," "Malcolm in the Middle" and "Charmed." Also on the chopping block is "7th Heaven," but lately there’s been talk it might be revived this fall when its network, the WB, merges with UPN to form the new CW.

Linwood Boomer, the creator of "Malcolm," says his show’s writers have known for a while this would be the last season for the Fox comedy. So, they’ve contemplated all year how to give seven characters a send-off in a satisfying way. (Malcolm goes to Harvard, for one thing.) Fans will get an idea of what the characters’ lives will be like in the future.

"But we also wanted to do an actual good episode," Boomer says. "So we don’t have anyone die, we don’t have anyone sent to the moon, we don’t have Dr. Phil come on. You want to make it special and interesting. At the same time, you don’t want to avoid doing all the stuff that [viewers] tune in to see."

Writers have one thing on their side — the stakes are low because so few finales have been regarded as classics.

"I don’t know anyone who likes the last ’Seinfeld,’ " Boomer says. "I’d say the jury’s split on who liked the last ’Cheers.’ Everyone sort of liked the last ’Raymond.’ "

Finales that stand out have been "normal" episodes with a little extra something added in at the end that lets viewers know, "if you never see them again, this is probably going to be what happens to them," Boomer says.

That was certainly true of the final "Mary Tyler Moore Show" ("Everyone loved the last ’Mary,’ " Boomer says), as well as the farewells for "Newhart" and "Six Feet Under" — all regarded as ideal series-enders.

Then there were those finales for such shows as "Buffy the Vampire Slayer" and "Angel" that were immensely satisfying to fans without getting much mainstream notice.

But, of course, there were the huge stinkers. "Roseanne," one of TV’s funniest series, ended by telling us that much of the series had been the fictional thoughts of the main character. "St. Elsewhere" wrapped up by claiming the whole series had been the idle musings of an autistic kid. Fans were justified in thinking they had had the rug pulled from under them.

The main reason this season’s exiting shows are going, going, gone is they’ve taken tumbles in the ratings. But Eric McCormack — who put the Will in "Will & Grace" — said the cast discussed some time ago that they should put a pretty bow on the show and go.

"The four of us just last year wanted to make sure that the show went out properly, you know, with a bang and not a whimper," McCormack said. "Sometimes, if you stay around one year too long, that’s obviously what happens. So we just wanted to protect these characters, and protect the legacy that [the writers] created."

Legacy is exactly what writers, actors and crew members are now pondering. Boomer remembers what it was like as a boy to watch a good show that got stuck in his brain forever. That’s the kind of good effect he hopes "Malcolm" has had, and will have in syndication — as long as the finale doesn’t ruin that legacy.

Bradley Whitford, who played Josh on "The West Wing," thinks back to the interactions he’s had with fans, and one little story comes to mind that illustrates how invested viewers feel about these exiting shows:

"Alan Greenspan once said to me — you know, with that face that is just trained not to express anything — he said that he was really upset that when our Fed chairman died, nobody cared. It wasn’t even the ’A’ story.’ "

Some older actors face the end with eloquent gratitude.

"It’s been the most fun I’ve had as an actor, and I’m glad that I’m old enough to appreciate it," says Stephen Collins, who plays the Rev. Eric Camden on "7th Heaven." "If I was younger, I probably wouldn’t have gotten what an amazing opportunity it was. I’m just glad people found it compelling."

And Martin Sheen — President Bartlet on "The West Wing" — knows the conclusion of his show is the end of a signature job:

"I’ll be lucky enough to have three jobs in my life that were earmarks for me. And that was Terry Malick’s ’Badlands,’ Francis Coppola’s ’Apocalypse Now’ and Aaron Sorkin and John Wells’ ’The West Wing.’"

Looking at the end of his fake presidency, Sheen hopes the depth of "The West Wing" is not forgotten.

"If we achieved some level of grace [for] the country, it would have been that we made the average American appreciate the level of service that 3 million government employees supply; that they are good and decent people [who] serve their country almost anonymously. And they do it out of a sense of patriotism and service."

That could be a legacy for "The West Wing" — as long as the finale doesn’t send anyone to the moon, or guest-star Dr. Phil, or end up as the meditations of an autistic boy.