Your salvia isn’t dying – it’s just doing what these plants do in the summer after they flower. They go to seed and the flowering stems get brown. Sometimes this browning is made worse by the plant getting hit too frequently with water. (Such as from an automatic irrigation system that comes on more than once a week.) So be sure that the plant is only getting watered ever 6 to 7 days. Cut off all the brown parts, leaving the lower foliage only. In the future you can do this right after it stops flowering, usually in early July. Then plant another perennial, or better yet an annual such as Profusion Zinnias or Blue Horizon Ageratum, near the plant so that those will have the flower power for the rest of the summer and into the fall.