Normally, by genetic selection of fastigiate cultivars. In gen Citrus, by pruning and tying the branches to the trunk to force them grow in a narrow angle. Citrus is not a fastigiate growing kind of tree. They always develop a broad canopy with time. Stems in a very acute angle will catch rotten fruits, fallen leaves and diseases
Using several narrow trees, instead a single wider tree, mean less fruits and more trunks/wood in the same space. Multigrafting a tree looks like a better solution