IF ALL'S FAIR in love and war, why not don your armor early this weekend and steal a march on Valentine's Day. There's more than one way to take aim at your favorite heart.

For starters, and a tax-deductible $25, you can send a singing valentine to the desk or door of that certain someone. Members of the Alexandria Choral Society or the American Cancer Society will perform a song of your choice and deliver a flower and a message.

On a more colorful note, Theodore's furniture store in Georgetown offers "Variations in Pink," a week-long program designed to help you create romance in your home. Valentine's Day kicks off early there at noon on Saturday with pink champagne, informal modeling, a catering competition -- where you can judge the most perfectly calculated romantic evening for two -- and an appearance by two romance novelists.

At the Washington Cathedral's Brass Rubbing Center, you can rub a valentine the right way from a wide selection of "love theme" 14th-century engravings.

The following activities could help you hit the mark with your valentine, so fire away.

SINGING VALENTINES -- Let cupid sing to your loved one. Within the District and Virginia, the Alexandria Choral Society will deliver your choice of a suitably sentimental song, posy and message for you. $25. 530-9596. The American Cancer Society will do the same in Montgomery County for the same tax- deductible fee. 933-9350. Deadline for placing orders is Monday.

BRASS RUBBINGS -- Daily through Feb. 14, 9:30 to 5, Washington Cathedral Brass Rubbbing Center, Massachusetts and Wisconsin Avenues NW. Create a unique valentine this year under the supervision of expert brass rubbers. $2.75. 364-9303.

VARIATIONS IN PINK, DESIGNS IN ROMANCE -- Saturday, noon to 5, Theodore's Contemporary Interiors, 2233 Wisconsin Ave., Georgetown. Find out how to bring romance into your home with appropriate food, fashion, furnishings and fantasy. Free. 656-5331.

VALENTINES FOR UNICEF -- Saturday, noon to 6, Prince George's Plaza Mall, 3500 East-West Highway at Belcrest Road, Hyattsville. Free entertainment featuring the Federal City String Band and the Velvet Monkeys. Raffle tickets and crafts; proceeds go to UNICEF African famine fund.

VICTORIAN VALENTINES -- Saturday and Sunday, noon to 4, Surratt House and Tavern, 9110 Brandywine Rd., Clinton, Maryland. See an exhibit of 19th- century valentines from private collections. $1 adults; senior citizens 75 cents; students 50 cents. 868-1121.

DANCE -- Saturday, 8:30 to midnight, St. Elizabeth's Church Hall, 919 Montrose Rd., Rockville. Join the Separated and Divorced Catholics for an evening of dancing, conversation and getting in the valentine mood. $5. 656- 0963 or 382-3686.

POLKA DANCE -- Saturday, 9 p.m. to 1, College Park American Legion Hall, 9218 Baltimore Blvd., College Park. The Melody Makers provide music for the Polish Club of College Park's sweetheart dance. Beer and munchies provided. $8.50. 345-9767.

VALENTINE'S BENEFIT -- Tuesday, 8 to closing, Tracks, 1111 First St. SE. Dance to the music of Philadelphia's Girlfriends; crafts on display. Proceeds for Sisterfire '85. $6. 488-3320 or 234-9308.

VALENTINE'S FLORAL ARRANGEMENT -- Wednesday, 7 to 8:30, Mt. Vernon Recreation Center, 2017 Belle View Blvd., Belle View. (Richmond Highway, Route 1 bearing right onto Route 629). Create your own floral arrangement. Bring scissors, knife and wire cutters; all other materials provided. $10. Reservations must be made by Monday. 768-3224.

ICE SKATING -- Thursday, 8:15 p.m. to 10:15, Mt. Vernon Recreation Center, 2017 Belle View Blvd., Belle View. Skate away with your valentine. Couples admitted for the price of one. $3 adults, $2.25 under 15, seniors $2. 768-3224.

VALENTINE'S MUSIC -- Thursday, 7:30, Baird Auditorium, Smithsonian Natural History Museum, 10th and Constitution NW. Pianist Bob Vigoda takes a musical look at the saucy and sentimental side of love from Richard Rodgers to Paul McCartney. Party follows concert in rotunda. $12 members of Smithsonian Resident Associate Program, $15 nonmembers. 357-3030.