For years we all were admonished to "waste not, want not," as we pushed heaps of spinach around the plate or begged off from another helping of turnips.

In December we recall the motto with pleasure, citing it as the reason we--dutifully--must finish all the Christmas cookies and pies.

Alas, comes January and we discover that in the avoidance of waste we have developed waist. Post-holiday plump.

The bleakness of winter is bleaker: Instead of being full of Christmas cheer, we are full of good resolve, diet and the solitude born of knowing that we cannot see our friends, for to meet is to eat.

But January needn't be a dreary month if you take your New Year's resolution to exercise and turn it into a party. According to the book Total Body Training by Richard H. Dominguez, M.D., with Robert Gajda (Charles Scribner's Sons), "Cross-country skiing is probably the best single form of aerobic exercise known to modern man." It also is among the most pleasant, and weather willing, it is easy to plan a cross-country skiing party that takes advantage of trails close to home.

A perfect trail is provided by Virginia's narrowest park, the Washington and Old Dominion Trail on the roadbed of the Washington and Old Dominion Railroad. You can enter the trail at Falls Church, where Lee Highway intersects Rte. 66, and follow it as far as your energy takes you. One hundred yards wide and 42 miles long, the trail ends at Leesburg in Loudon County. To check snow conditions on the trail, call 278-8880.

Another spot for cross-country skiing in Virginia is Prince William Forest Park, 17 miles south of 495 on I-95 (state Rte. 619). Follow signs from the entrance of the park to the visitors' center, where you can get information on the trails. For snow conditions, call 703-221-2104.

In Montgomery County, you can cross-country ski in the Stream Valley parks. There is the Northwest Branch, which runs along New Hampshire Avenue between the Beltway and Rte. 108; Sligo Creek, off Sligo Creek Parkway between Takoma Park and University Boulevard; Rock Creek, along Beach Drive or off Muncaster Mill Road; Paint Branch, off Randolph or Fairland roads between New Hampshire Avenue and Rte. 29, and Little Seneca, north of Boyds, Md., with entrances off Routes 270 and 121.

For information about snow conditions, call Montgomery County Park Police, 949-3010. For a guide to Montgomery County parks, call 565-7401.

To keep the focus on eliminating waist, ask everyone to gather at your house, bringing thermoses and knapsacks. Before you set out, make sure everyone has this take-along lunch: oxtail soup (2 cups, 200 calories), bread (65 calories a slice), an apple (117 calories) and a 1 1/3-oz. wedge of Camembert (115). Later you can stop along the trail, take off your skis and bask in virtue and friendship.

Ice skating is less strenuous than cross-country skiing but still good exercise. Many local rinks are not only open weekends, but have moonlight evening hours to provide a romantic skating party.

Least strenuous of all, partaking more of the virtues of fresh air than of exercise, would be a bird-watching party. The Audubon Naturalist Society can provide a leader who will teach you the difference between a cardinal and a crow. The society is currently taking its winter census, gathering data on the winter bird population in the D.C. area. If you'd like to join them in a bird count, three are scheduled Jan. 8:

* At the National Arboretum (call Dave Czaplak, 529-9499).

* At Glover Archbold Park (call Shirley Briggs, 657-8685).

* At Cabin John Island in Maryland (call Richard Gauthey, 451-5047).

Afterward, you all can go home and peck at your food.