Answer: Pines can be made to grow denser by breaking off just a bit of the "candle" at each branch tip very early each spring before they have begun to expand much. Each broken candle will cause that branch to grow denser rather than elongated and spindly. This is why many commercial Christmas trees are sheared regularly in order to give such a full (and somewhat unnatural) look.
You might also want to make sure they are receiving the fertility and water that they need to grow vigorously.
Q&A Library Searching Tips