Having succeeded in its novel business by trying very hard, Burn Brae Dinner Theater begins its tenth year with "How to Succeed in Business Without Really Trying." Rodney Fayman, who's played J. Pierpont Finch before, is smooth and sly and also has staged the '62 Pulitzer winner with skillful ease.

Like Finch, who went from window-washer to leadership at World Wide Wickets, Burn Brae has risen to elegance from a humbel start as the area's first dinner theater. Additions have eased the seating capacity up to an uncrowded, five-tiered 350. "As large as we intend to go," says co-producer Bernard T. Levin.

There's the former country club's now spruced-up big pool for a swim before dinner. Under actor-chef John Preece, the menu is lavish and varied. It's a safe bet that most of the decade's profits have been poured back into the plant on Rte. 29, Burtonsville, Md.

In Shepherd Mead's satire about the mores of the business jungle, life once again has copied art.

Mead's guidebook to one-upsmanship might easily have influenced such later ignobles as Haldeman, Ehrlichman, Dean and Ray. Mindlessly ambitious, Finch, Frump, Twimble and had no more of a clue about World Wide Wicket's mission than the White House Mafia had about governing the United States. Ray took her words about knowing nothing of secretarial skills straight from Hedy La Rue.

Frank Loesser's lyrics and score are the equal of his earlier "Guys and Dolls" with such numbers as "A Secretary Is Not a Toy," "Grand Old Ivy," "I Believe in You," "Happy to Keep His Dinner Warm" and "The Brotherhood of Man."

Easy and comical, Fayman has a better voice than most Finches. Eileen Wiswell is a lovely, poised Rosemary and, as Bud Frump, Bob Arnold is most amusingly wicked. Jed Springfield, Catherine Blaine, Preece and Steve Wappel are others in the large cast, with John Sichina as musical director. Reservations at 384-5800.