Fall-blooming salvias such as leucantha should be pruned during the summer to keep them compact, reducing the need for staking. To make the job easier, use hedging shears, and remove only the spent flowers and a few inches of stem below. Any of this type of pruning should be completed prior to September 1, since flower buds begin forming about that time.
Note from Parker County Master Gardener: As for the leucantha, it has to be cut back completely to ground as does Victoria blue salvia because it dies to ground.