Genetics first- Indica background promotes tighter buds.
Next, light intensity and type. Intense light in the spectrum produced by a HPD lamp is ideal for promoting bud density.
Last, fertilizers, including carbon. The grower must provide ample Phosphorous and Carbon for the plant to put on bud mass.

Density is not always a good thing, though. For example, strains meant to be grown outdoors in areas of frequent rainfall or higher humidity must have fluffy buds to dry out quickly before rot sets in.