From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156583-203511
Question: How do I prune evergreen trees to make them fill out? They
are 8 to 10 feet tall. I'd like to look more like Christmas trees.
Answer: Christmas trees are sheared annually starting when they're
very young, with an overall trim to achieve those "fat" shapes.
Shearing is generally not the best method for long-term health of most
plants, but since these trees are destined to be cut young, it is
acceptable. Many types of evergreen shrubs and trees need little or no
pruning to keep them in prime condition. Here are some general
guidelines for pruning needled evergreens: Prune those that produce
side buds on the shoots, such as spruce, fir and Douglas-fir, by
cutting the tips back to the desired length in late winter or early
spring when buds are dormant. In plants which produce few side buds,
such as pines, pruning may be accomplished by pinching or snapping off
with the fingers a portion of the young growth "candles" that form in
late spring and early summer. The proper time to prune is when the
"candles" snap off easily and cleanly. Avoid cutting these plants with
a knife or shears, since this method invariably injures some of the
needles and later gives the plant a general brown appearance. This
type of pruning will result in bushier plants, but probably not quite
as bushy as Christmas trees.