Here's the solution to every don't in your do.

 Hair is evidence. It can tell your boss if you did drugs at any point within the past 90 days, the homicide detectives if you dropped your DNA at the murder scene, and your houseguests if you ever bother to clean the bathtub. But, more important, your collective strands are the smoking gun of poor grooming.

Whether it's flaking or thinning, gray or greasy, troubled hair is an instant indictment of a man's appearance. But we can fix things before you face a jury of your peers (especially the well-endowed, blonde peers). Think of us as the reasonably priced yet slightly shady counsel that'll tell you how to doctor the evidence.

This is what you should do when your hair is . . .

Thinning

You're awake, but your hair follicles have decided to sleep in. Blame dihydrotestosterone (DHT), a hormone that, in excess, lengthens the resting phase of your hair's growth cycle and eventually causes it to jump ship.

* How to save your scalp: Try a double-barreled approach. In the morning, apply a 5% minoxidil solution. Minoxidil is still the only over-the-counter product FDA approved to help stop the shedding and grow new hair. At night, try a new spray called FNS (Follicle Nutrient Serum). While FNS ($60) is not yet FDA approved, a study showing that the spray stopped hair loss in men after 3 weeks has prompted researchers to suggest giving it a try.

"It contains a natural insulin substitute that allows nutrients to get into the cells of the follicles and stimulate hair growth," says Peter Elias, M.D., a professor of dermatology at the University of California at San Francisco and an editor at the Journal of Investigative Dermatology. FNS is available at Saks Fifth Avenue and Nordstrom, as well as through www.osmotics.com.

Snowing

If it makes you feel any better, don't call it dandruff, call it seborrhea, an inflammation of the scalp -- possibly triggered by yeast and bacteria -- that causes redness, itching, and those telltale flakes of skin.

* How to save your scalp: Gang-tackle your dand--, er, seborrhea. "One shampoo just won't do it," says Audrey Kunin, M.D., a professor of dermatology at the University of Kansas and founder of Derma doctor.com. Buy one with both antifungal and antibacterial ingredients (like Carmol Deep Cleaning), one with zinc (like Head & Shoulders) to calm the inflammation, and one with salicylic acid (like Neutrogena T/Sal) to cut through the crust. Use one shampoo for a few days and then switch. If you're still leaving a wake of white after two months, see a dermatologist. He'll probably prescribe a steroid-based shampoo, such as Capex.

Butchered

Your barber just quit smoking.

* How to save your scalp: Have another barber cut that mistake on your head even shorter, says Dale Sheffield, director of the Roffler hairstyling college in Marietta, Georgia. "Usually a bad haircut is uneven, so cutting the hair to its shortest length will balance it out." Next, work some styling gel or pomade into your hair for a tousled look. "This will help hide a bad haircut, even if it's not the style you're used to," says Jesse McCorkell, a technical advisor for the Great Clips for Hair chain.

Greasy

Some heads are simply programmed to pump out more sebum -- our body's version of Valvoline -- than others.

* How to save your scalp: Wash the dishes and then your hair. "Nothing beats a little dishwashing liquid for emergency sebum control," says Lynn Symonds, author of The Haircutting School Instruction Book. "But it's too drying to use every day." For daily mop-up duty, use Aqua Glycolic shampoo and body cleanser. "The glycolic acid cuts through and dissolves much of the extra oil," says Dr. Kunin.

Dry

Maybe you're short on sebum. Or you swim a lot. Or you like to blow-dry the heck out of your hair. Whatever the cause, the result is the same: a parched pate.

* How to save your scalp: Use a conditioner -- but don't follow the directions. After you get out of the shower, put a dime-size dab in your palm, rub your hands together, and then run them through your hair. "This will quickly moisturize your hair and make it more manageable in the short term," says Symonds.

For long-term relief, use a shampoo that says "gentle" or "mild" on the bottle and look for the nondrying ingredient sodium laureth sulfate, says Frank Cunningham, a hair and scalp specialist in Manchester, England.

Cowlicked

Your hair is giving you the finger: One tuft is growing in the wrong direction, refusing to be pomaded into place.

* How to save your scalp: Hit that cowlick and hit it hard with a relaxer, what women use to straighten curls, says McCorkell. Think of a relaxer as a reverse perm -- it breaks down the structure of the hair so you can straighten it. You simply apply the stuff with a comb or brush, leave it in for 5 to 10 minutes, then comb the cowlick straight. Reapply every 3 months. McCorkell's pick for a good relaxer: Rusk's Anti-Curl. "Unlike most relaxers, it won't make your hair stiff," he says.

Sticky

You already know bubble gum belongs in your mouth and not in your hair. Now tell your 5-year-old.

* How to save your scalp: Swallow your pride and reach for the antidote to bubble-gum head: peanut butter. "Apply a small amount to the affected area and rub it in," says Sheffield. "The oil will soften the gum and let it slip out of the hair." Shampoo afterward. And don't forget: Use smooth-style peanut butter, not the chunky kind.

Graying

A single gray hair at age 32 can be ignored -- or pulled out. But 157 of them? Time to dye. See, once your body slows its production of the body pigment melanin and your hair loses its luster, there's no going back, only covering up.

* How to save your scalp: Skip the Grecian Formula your first time out and pick up a women's semipermanent dye like Clairol's Loving Care instead, recommends McCorkell. Why the girlie dye? In case you screw up.

A men's semipermanent dye will take 12 to 24 shampooings to wash out, while the women's will take about six.

"Pick a color that's a shade lighter than your natural color -- if it's too light, it's not going to cover the gray; too dark and people will be able to tell you dyed your hair," says McCorkell. Once you find the right hue, pick it up in a longer-lasting men's dye.

Missing

If crop circles suddenly appear in your hair, don't blame aliens; you've been visited by alopecia areata, a disorder in which your immune system decides hair follicles are enemies and kills them in dime- or quarter-size patches.

* How to save your scalp: Buy 1% hydrocortisone cream and rub it into each bald circle. This may calm the inflammation enough to allow the follicles to regrow, says Richard Greene, M.D., of the Skin and Cancer Associates dermatology group in Florida. Didn't work? See a dermatologist, who will most likely prescribe topical or injectable steroids. They're effective in 80% of cases, says Dr. Greene.

A Honey of a Comb

There's only one touch-up tool that a man with class should shove in his back pocket, and that's a comb. Already own one of those black 99-cent numbers? That's a garden rake, not a comb. "The teeth should be smooth and round on the bottom, otherwise they'll scratch your scalp," says Peppe Baldo, owner of Peppe Hairstylists at the Plaza Hotel in New York City. Our pick is the Mason Pearson C5 pocket comb ($9). Handmade in Switzerland from polished acrylic, it has teeth that won't bite, plus a classic tortoise-shell finish that'll accent any suit. Call (800) 645-6503 to find a dealer near you.