Hair today, gone tomorrow. There is more to Farrah Fawcett-Majors than her hair, doll manufacturers are discovering. Hasbro, major toy manufacturers who also make Don't Cry Baby doll and Mr. Potato Head, have come up with Charlie's Angels Dolls - three Barbie Doll-size dolls cum customes and skateboard and even hideaway house each sold separately. Of course, Fawcett-Majors is outselling her TV partners two to one. According to Carole MacGilvray, the company used WWD and the fashion slicks to get their ideas for the fantasy costumes to replace the jumpsuit the three dolls wear. Toys-R-Us will have the dolls next month.

Based on the success of the Cher doll, Mego Corp. which sold 2 million Cher dolls last year, has enlisted Bob Mackie, Hollywood designer who makes clothes for Cher and others, to make costumes and makeup kit to be sold for dolling up their FFM doll.

Just who created the first FFM hairstyle is a knotty question. Hugh York at Elizabeth Arden says he started it. "It's not a layer or shag, but a special kind of graduation all around," he says. "It takes hot curlers, rollers, lots of pampering and care."

Elizabeth Taylor has put her stamp on a collection of diamonds to be sold unmounted at first, and as jewelry designs later. The Elizabeth Taylor diamond comes with an individual diamond analysis, a "gemprint" achieved by photographing the reflection of the stone. The metal slide package has the Taylor profile stamped on a gold metal coin and her signature as well.

Jantzen sportswear is crediting President Carter rather than the cold weather with boosting sweater sales - as much as 43 per cent for that firm alone, and has registered "Jimmy Cartergan" to promote future cardigan sweater sales.

Walt Frazier has let the fact that he was named to a best-dressed list to go to his head. The New York Knicks star has a personal hatter, Bob Posey of Dobbs Hats, and almost enough hats to go with each of his hundred or so wardrobe changes. (Dobbs hats are at Cavalier Men's Shops and Brodt's.)

Their names are not on labels yet, but it may not take long for two Washington area high school graduates now at design schools. Steven Leavitt, a Herndon High graduate now at Pratt Institute in New York and Sherwin Derby, who graduated from Osborn High in Manassas, were among the top 15 winners in the America's Next Great Designer Awards. The contest, sponsored by the International Ladies Garment Workers Union, drew 6,000 sketches from student contestants and was judged by leading designers from Seventh Avenue and fashion editors.

Fat, 40or flushed? There are books on these subjects this month. For the over 40, Maxine Daley and Barbara Lochner, in their book "How To Get a Man After You're Forty" suggest, among other things, always wear long sleeves, steer clear of ruffled necklines and don't wear slips. To encourage individual style, they conclude, it's dreary to be perfect, like leaving the cellophane on lampshades.

"Body Hot Spots" by dale Guthrie is an explicit discussion of body communication.

A perfect size 22 1/2, Stella Jolles Reichman, author of "Great Big Beautiful Doll," thinks one day soon, skinny is bound to be unfashionable. But for those like herself, who aren't too skinny, Reichman offers a mildly convincing pep talk and then suggests sound exercises (accompanied by pictures of her zaftig self), plus makeup, hair and clothing tips. She is not locked into the expected how-tos for large women, but offers some rather good ideas including adapting army-navy surplus clothing.